[aarch64] Make common aarch64 options target-dependent

This patch replaces the "Common" attribute in some aarch64 options with "Target"
to make them target-dependent. This doesn't affect their behaviour or how they
are documented with "--help", but does mean that they follow the option
specification that target-specific options are given the "Target" attribute.

Tested by ensuring the changed options are still accepted as before and that
they aren't invokable from an invalid target. Bootstrapped and regression tested
on aarch64-none-elf with ... regressions.

2018-08-16  Sam Tebbs  <sam.tebbs@arm.com>

	* config/aarch64/aarch64.opt (mlow-precision-recip-sqrt)
	(mlow-precision-sqrt, mlow-precision-div, mverbose-cost-dump): Replace
	"Common" with "Target".

From-SVN: r263578

gcc/ChangeLog

+2018-08-16  Sam Tebbs  <sam.tebbs@arm.com>
+
+	* config/aarch64/aarch64.opt (mlow-precision-recip-sqrt)
+	(mlow-precision-sqrt, mlow-precision-div, mverbose-cost-dump): Replace
+	"Common" with "Target".
+
 2018-08-15  Uros Bizjak  <ubizjak@gmail.com>
 
 	* config/i386/i386.opt (mmitigate-rop): Mark as deprecated.

gcc/config/aarch64/aarch64.opt

@@ -167,20 +167,20 @@ EnumValue
 Enum(aarch64_ra_sign_scope_t) String(all) Value(AARCH64_FUNCTION_ALL)
 
 mlow-precision-recip-sqrt
-Common Var(flag_mrecip_low_precision_sqrt) Optimization
+Target Var(flag_mrecip_low_precision_sqrt) Optimization
 Enable the reciprocal square root approximation.  Enabling this reduces
 precision of reciprocal square root results to about 16 bits for
 single precision and to 32 bits for double precision.
 
 mlow-precision-sqrt
-Common Var(flag_mlow_precision_sqrt) Optimization
+Target Var(flag_mlow_precision_sqrt) Optimization
 Enable the square root approximation.  Enabling this reduces
 precision of square root results to about 16 bits for
 single precision and to 32 bits for double precision.
 If enabled, it implies -mlow-precision-recip-sqrt.
 
 mlow-precision-div
-Common Var(flag_mlow_precision_div) Optimization
+Target Var(flag_mlow_precision_div) Optimization
 Enable the division approximation.  Enabling this reduces
 precision of division results to about 16 bits for
 single precision and to 32 bits for double precision.
@@ -212,7 +212,7 @@ Target RejectNegative Joined Enum(sve_vector_bits) Var(aarch64_sve_vector_bits)
 -msve-vector-bits=N	Set the number of bits in an SVE vector register to N.
 
 mverbose-cost-dump
-Common Undocumented Var(flag_aarch64_verbose_cost)
+Target Undocumented Var(flag_aarch64_verbose_cost)
 Enables verbose cost model dumping in the debug dump files.
 
 mtrack-speculation