libphobos: RISC-V: Fix soft-float build errors with IEEE exception flags

Fix assembly errors:

.../libphobos/src/std/math.d: Assembler messages:.../libphobos/src/std/math.d:4773: Error: unrecognized opcode `frflags a0'.../libphobos/src/std/math.d:4856: Error: unrecognized opcode `fsflags a5'.../libphobos/src/std/math.d:4856: Error: unrecognized opcode `fsflags a5'.../libphobos/src/std/math.d:4773: Error: unrecognized opcode `frflags a0'.../libphobos/src/std/math.d:5549: Error: unrecognized opcode `fscsr a5'.../libphobos/src/std/math.d:5456: Error: unrecognized opcode `frcsr a5'.../libphobos/src/std/math.d:5456: Error: unrecognized opcode `frcsr a5'.../libphobos/src/std/math.d:5549: Error: unrecognized opcode `fscsr a5'.../libphobos/src/std/math.d:5456: Error: unrecognized opcode `frcsr a5'.../libphobos/src/std/math.d:5549: Error: unrecognized opcode `fscsr a0'.../libphobos/src/std/math.d:5456: Error: unrecognized opcode `frcsr a0'.../libphobos/src/std/math.d:5456: Error: unrecognized opcode `frcsr a0'.../libphobos/src/std/math.d:5549: Error: unrecognized opcode `fscsr s2'make[8]: *** [Makefile:1119: std/math.lo] Error 1

triggered with the RISC-V lp64 multilib in a GCC build configured with 
`--enable-multilib --enable-languages=all --target=riscv64-linux-gnu'. 
This is due to unconditional explicit use of F extension instructions 
within inline assembly, to access IEEE exception flags.  The use of 
these instructions is not allowed when building for a soft-float ABI.

Correct the problem by wrapping said inline assembly into a conditional 
such that if `D_SoftFloat' is true, then reads from IEEE exception flags 
return 0 and writes are ignored instead, complementing r270522 
("libphobos: Add D support for RISC-V Linux"), which is an updated 
version of <https://gcc.gnu.org/ml/gcc-patches/2019-04/msg00325.html>, 
where the problematic code has originated from.

libphobos/ChangeLog:

2019-05-02  Maciej W. Rozycki  <macro@wdc.com>

	* std/math.d (IeeeFlags.getIeeeFlags): Handle RISC-V soft-float ABI.
	(IeeeFlags.resetIeeeFlags): Likewise.
	(FloatingPointControl.getControlState): Likewise.
	(FloatingPointControl.setControlState): Likewise.

From-SVN: r270815

libphobos/ChangeLog

+2019-05-02  Maciej Rozycki  <macro@wdc.com>
+
+	* std/math.d (IeeeFlags.getIeeeFlags): Handle RISC-V soft-float ABI.
+	(IeeeFlags.resetIeeeFlags): Likewise.
+	(FloatingPointControl.getControlState): Likewise.
+	(FloatingPointControl.setControlState): Likewise.
+
 2019-04-25  Iain Buclaw  <ibuclaw@gdcproject.org>
 
 	PR d/90250

libphobos/src/std/math.d

@@ -4767,6 +4767,10 @@ private:
             }
             else version (RISCV_Any)
             {
+                version (D_SoftFloat)
+                    return 0;
+                else
+                {
                     uint result = void;
                     asm pure nothrow @nogc
                     {
@@ -4774,6 +4778,7 @@ private:
                     }
                     return result;
                 }
+            }
             else
                 assert(0, "Not yet supported");
         }
@@ -4850,12 +4855,17 @@ private:
             }
             else version (RISCV_Any)
             {
+                version (D_SoftFloat)
+                    return;
+                else
+                {
                     uint newValues = 0x0;
                     asm pure nothrow @nogc
                     {
                         "fsflags %0" : : "r" newValues;
                     }
                 }
+            }
             else
                 assert(0, "Not yet supported");
         }
@@ -5450,6 +5460,10 @@ private:
             }
             else version (RISCV_Any)
             {
+                version (D_SoftFloat)
+                    return 0;
+                else
+                {
                     ControlState cont;
                     asm pure nothrow @nogc
                     {
@@ -5457,6 +5471,7 @@ private:
                     }
                     return cont;
                 }
+            }
             else
                 assert(0, "Not yet supported");
         }
@@ -5544,11 +5559,16 @@ private:
             }
             else version (RISCV_Any)
             {
+                version (D_SoftFloat)
+                    return;
+                else
+                {
                     asm pure nothrow @nogc
                     {
                         "fscsr %0" : : "r" (newState);
                     }
                 }
+            }
             else
                 assert(0, "Not yet supported");
         }