LoongArch: Adjust C++ multilib header layout.

For LoongArch, the toplevel library build is always aliased to
one of the multilib variants.  This patch installs it with the
actual MULTISUBDIR (instead of ".") so that the headers can be
reached by the compiler.

This patch is an update of
https://gcc.gnu.org/pipermail/gcc-patches/2023-September/629435.html

libstdc++-v3/ChangeLog:

	* configure.host: Register t-loongarch in tmake_file.
	* config/cpu/loongarch/t-loongarch: New file.  Manually refresh
	MULTISUBDIR with $(shell $(CXX) --print-multi-directory).