Make C2X imply -fno-fp-int-builtin-inexact.

Make C2X imply -fno-fp-int-builtin-inexact.

Since TS 18661-1 has been integrated into C2X, this patch makes C2X
imply -fno-fp-int-builtin-inexact.

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

gcc:
	* doc/invoke.texi (-ffp-int-builtin-inexact): Document
	-fno-fp-int-builtin-inexact default for C2X.

gcc/c-family:
	* c-opts.c (c_common_post_options): Set
	-fno-fp-int-builtin-inexact for C2X.

gcc/testsuite:
	* gcc.dg/torture/builtin-fp-int-inexact-c2x.c: New test.

From-SVN: r276686