libstdc++: [_GLIBCXX_INLINE_VERSION] Fix tests failures

libstdc++: [_GLIBCXX_INLINE_VERSION] Fix tests failures

Adapt testsuite v3_target_compile to strip version namespace from compiler
output so that dg-error and dg-warning directives do not need to consider it.

libstdc++-v3/ChangeLog:

	* testsuite/lib/libstdc++.exp (v3_target_compile): Strip version namespace
	from compiler output.
	* testsuite/20_util/function/cons/70692.cc: Remove now useless __8 namespace
	pattern.
	* testsuite/23_containers/map/48101_neg.cc: Likewise.
	* testsuite/23_containers/multimap/48101_neg.cc: Likewise.

Co-authored-by: Jonathan Wakely <jwakely@redhat.com>
(cherry picked from commit 8709d6a17830c8a9f48cb3ac6dfc6af76f2e1e81)