libatomic, libgomp, libitc: Fix bootstrap [PR70454]

The recent changes to error on mixing -march=i386 and -fcf-protection broke
bootstrap.  This patch changes lib{atomic,gomp,itm} configury, so that it
only adds -march=i486 to flags if really needed (i.e. when 486 or later isn't
on by default already).  Similarly, it will not use ifuncs if -mcx16
(or -march=i686 for 32-bit) is on by default.

2021-01-15  Jakub Jelinek  <jakub@redhat.com>

	PR target/70454
libatomic/
	* configure.tgt: For i?86 and x86_64 determine if -march=i486 needs to
	be added through preprocessor check on
	__GCC_HAVE_SYNC_COMPARE_AND_SWAP_4.  Determine if try_ifunc is needed
	based on preprocessor check on __GCC_HAVE_SYNC_COMPARE_AND_SWAP_16
	or __GCC_HAVE_SYNC_COMPARE_AND_SWAP_8.
libgomp/
	* configure.tgt: For i?86 and x86_64 determine if -march=i486 needs to
	be added through preprocessor check on
	__GCC_HAVE_SYNC_COMPARE_AND_SWAP_4.
libitm/
	* configure.tgt: For i?86 and x86_64 determine if -march=i486 needs to
	be added through preprocessor check on
	__GCC_HAVE_SYNC_COMPARE_AND_SWAP_4.