libatomic: Consider '--with-build-sysroot=[...]' for target libraries'...

libatomic: Consider '--with-build-sysroot=[...]' for target libraries' build-tree testing (instead of build-time 'CC' etc.) [PR109951]

Similar to commit fb5d27be
"libgomp: Consider '--with-build-sysroot=[...]' for target libraries' build-tree testing (instead of build-time 'CC' etc.) [PR91884, PR109951]",
this is commit 5ff06d76
"libatomic/test: Fix compilation for build sysroot" done differently,
avoiding build-tree testing use of any random gunk that may appear in
build-time 'CC'.

	PR testsuite/109951
	libatomic/
	* configure.ac: 'AC_SUBST(SYSROOT_CFLAGS_FOR_TARGET)'.
	* Makefile.in: Regenerate.
	* configure: Likewise.
	* testsuite/Makefile.in: Likewise.
	* testsuite/lib/libatomic.exp (libatomic_init): If
	'--with-build-sysroot=[...]' was specified, use it for build-tree
	testing.
	* testsuite/libatomic-site-extra.exp.in (GCC_UNDER_TEST): Don't
	set.
	(SYSROOT_CFLAGS_FOR_TARGET): Set.