[Patch][libstdc++.exp] Update the usage of cached result.

[Patch][libstdc++.exp] Update the usage of cached result.

This patch replaces the usage of cached results with a global dictionary.

Additionally, check_v3_target_namedlocale is updated to check on every variant.
Originally, it is only checked once.

gcc/libstdc++-v3/:
2018-11-16  Renlin Li  <renlin.li@arm.com>
	    Tejas Belagod  <tejas.belagod@arm.com>

	testsuite/lib/libstdc++.exp (check_v3_target_prop_cached): New proc.
	(check_v3_target): Use check_v3_target_prop_cached.

Co-Authored-By: Tejas Belagod <tejas.belagod@arm.com>
From-SVN: r266209