libffi: 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 a0b48358
"libffi/test: Fix compilation for build sysroot" done differently,
avoiding build-tree testing use of any random gunk that may appear in
build-time 'CC', 'CXX'.

	PR testsuite/109951
	libffi/
	* configure.ac: 'AC_SUBST(SYSROOT_CFLAGS_FOR_TARGET)'.
	<local.exp>: Don't set 'CC_FOR_TARGET', 'CXX_FOR_TARGET', instead
	set 'SYSROOT_CFLAGS_FOR_TARGET'.
	* Makefile.in: Regenerate.
	* configure: Likewise.
	* include/Makefile.in: Likewise.
	* man/Makefile.in: Likewise.
	* testsuite/Makefile.in: Likewise.
	* testsuite/lib/libffi.exp (libffi_target_compile): If
	'--with-build-sysroot=[...]' was specified, use it for build-tree
	testing.