[clang] Fall back on Android triple w/o API level for runtimes search

Clang searches for runtimes (e.g. libclang_rt*) first in a
subdirectory named for the target triple (corresponding to
LLVM_ENABLE_PER_TARGET_RUNTIME_DIR=ON), then if it's not found uses
.../lib/<os>/libclang_rt* with a suffix corresponding to the arch and
environment name.

Android triples optionally include an API level indicating the minimum
Android version to be run on
(e.g. aarch64-unknown-linux-android21). When compiler-rt is built with
LLVM_ENABLE_PER_TARGET_RUNTIME_DIR=ON this API level is part of the
output path.

Linking code built for a later API level against a runtime built for
an earlier one is safe. In projects with several API level targets
this is desireable to avoid re-building the same runtimes many
times. This is difficult with the current runtime search method: if
the API levels don't exactly match Clang gives up on the per-target
runtime directory path.

To enable this more simply, this change tries target triple without
the API level before falling back on the old layout.

Another option would be to try every API level in the triple,
e.g. check aarch-64-unknown-linux-android21, then ...20, then ...19,
etc.

Differential Revision: https://reviews.llvm.org/D115049

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