Similar to commit fb5d27be27
"libgomp: Consider '--with-build-sysroot=[...]' for target libraries' build-tree testing (instead of build-time 'CC' etc.) [PR91884, PR109951]",
this is commit 5ff06d762a
"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.