Various built-in functions that GCC has as extensions are now standard
functions in C2x.  This patch adds DEF_C2X_BUILTIN and uses it to mark
them as such.  Some of the so-marked functions were previously
DEF_EXT_LIB_BUILTIN, while some DFP ones were DEF_GCC_BUILTIN
(i.e. __builtin_* only); both sets become DEF_C2X_BUILTIN.  This in
turn requires flag_isoc2x to be defined in various front ends using
builtins.def.

As the semantics of the built-in functions should already be tested,
the tests added only verify that they are declared in C2x mode but not
in C11 mode.  The test of DFP built-in functions being declared for
C2x goes in gcc.dg/dfp/, as while such built-in functions currently
don't depend on whether DFP is supported, that looks like a bug to me
(see bug 91985), so it seems best for the tests not to depend on
exactly how that bug might be fixed.

Bootstrapped with no regressions on x86_64-pc-linux-gnu.

gcc:
	* builtins.def (DEF_C2X_BUILTIN): New macro.
	(exp10, exp10f, exp10l, fabsd32, fabsd64, fabsd128, nand32)
	(nand64, nand128, roundeven, roundevenf, roundevenl, strdup)
	(strndup): Use DEF_C2X_BUILTIN.
	* coretypes.h (enum function_class): Add function_c2x_misc.

gcc/ada:
	* gcc-interface/utils.c (flag_isoc2x): New variable.

gcc/brig:
	* brig-lang.c (flag_isoc2x): New variable.

gcc/lto:
	* lto-lang.c (flag_isoc2x): New variable.

gcc/testsuite:
	* gcc.dg/c11-builtins-1.c, gcc.dg/c2x-builtins-1.c,
	gcc.dg/dfp/c2x-builtins-dfp-1.c: New tests.

From-SVN: r276588