libgomp: Consider '--with-build-sysroot=[...]' for target libraries' build-tree testi...

libgomp: Consider '--with-build-sysroot=[...]' for target libraries' build-tree testing (instead of build-time 'CC' etc.) [PR91884, PR109951]

This is commit c8e759b4215ba4b376c9d468aeffe163b3d520f0 (Subversion r279708)
"libgomp/test: Fix compilation for build sysroot" and follow-up
commit 749bd22ddc50b5112e5ed506ffef7249bf8e6fb3
"libgomp/test: Remove a build sysroot fix regression" done differently,
avoiding build-tree testing use of any random gunk that may appear in
build-time 'CC', 'CXX', 'FC'.

	PR testsuite/91884
	PR testsuite/109951
	libgomp/
	* configure.ac: Revert earlier changes, instead
	'AC_SUBST(SYSROOT_CFLAGS_FOR_TARGET)'.
	* Makefile.in: Regenerate.
	* configure: Likewise.
	* testsuite/Makefile.in: Likewise.
	* testsuite/lib/libgomp.exp (libgomp_init): Remove
	"Fix up '-funconfigured-libstdc++-v3' in 'GXX_UNDER_TEST'" code.
	If '--with-build-sysroot=[...]' was specified, use it for
	build-tree testing.
	* testsuite/libgomp-site-extra.exp.in (GCC_UNDER_TEST)
	(GXX_UNDER_TEST, GFORTRAN_UNDER_TEST): Don't set.
	(SYSROOT_CFLAGS_FOR_TARGET): Set.
	* testsuite/libgomp.c++/c++.exp (lang_source_re)
	(lang_include_flags): Set for build-tree testing.
	* testsuite/libgomp.oacc-c++/c++.exp (lang_source_re)
	(lang_include_flags): Likewise.

Co-authored-by: Chung-Lin Tang <cltang@codesourcery.com>