[libcxx][CI] Set Arm triples to match native clang build's default

We were using:
armv8-linux-gnueabihf
But for a native clang build the default target is:
armv8l-linux-gnueabihf

(ditto for v7)

Add the "l" to the target triples and update the one test
that is unsupported to look for the various possible names.

armv(7 or 8)(m or l, optionally)

The UNSUPPORTED does not include aarch64 because aarch64 Linux
(and others that follow Arm's AAPCS64) use quad precision for
long double where arm64 (darwin) does not:
https://developer.apple.com/documentation/xcode/writing-arm64-code-for-apple-platforms
https://github.com/ARM-software/abi-aa/blob/main/aapcs64/aapcs64.rst#811arithmetic-types

Reviewed By: rovka

Differential Revision: https://reviews.llvm.org/D119948