libgomp/test: Remove a build sysroot fix regression

libgomp/test: Remove a build sysroot fix regression

Fix a problem with commit c8e759b4215b ("libgomp/test: Fix compilation
for build sysroot") that caused a regression in some standalone test
environments where testsuite/libgomp-test-support.exp is used, but the
compiler is expected to be determined by `[find_gcc]', and set the
GCC_UNDER_TEST TCL variable in testsuite/libgomp-site-extra.exp instead.

	libgomp/
	* configure.ac: Add testsuite/libgomp-site-extra.exp to output
	files.
	* configure: Regenerate.
	* testsuite/libgomp-site-extra.exp.in: New file.
	* testsuite/libgomp-test-support.exp.in (GCC_UNDER_TEST): Remove
	variable.
	* testsuite/Makefile.am (EXTRA_DEJAGNU_SITE_CONFIG): New
	variable.
	* testsuite/Makefile.in: Regenerate.