[libc++][dsl] Run checks for locale names aliases using a single %exec

This changes the checking for available locales to use one program that
iterates over argv to test multiple locale names instead of checking each
name with a separate executable.

This massively speeds up running individual tests using an SSH executor
(it can take up to 10 seconds to compile and run a single test in some
emulated environments) in case no locales are installed since then all
fallback names are tested idividually. But even on a native machine
this reduces the libc++ lit startup time by ~1-2 second for me on a machine
that does not have locale data installed.

Reviewed By: #libc, ldionne
Differential Revision: https://reviews.llvm.org/D88884