libstdc++: Replace hyphens in effective target keywords

libstdc++: Replace hyphens in effective target keywords

An effective target like foo-bar-baz will match a target selector of
*-*-* and cause problems in the testsuite. Several libstdc++ et keywords
are of the form foo-bar, which could still be a problem for *-*
selectors.

Replace hyphens with underscores in the et keywords "debug-mode",
"cxx11-abi", etc.

libstdc++-v3/ChangeLog:

	* testsuite/lib/libstdc++.exp: Rename effective target keywords
	to avoid dashes in the name.
	* testsuite/*: Update effective targe keywords.