third_party/stm32cube: Improve clang detection
third_party/stm32cube: Improve clang detection

Makes clang vs gcc detection more robust in stm32cube.

The stm32cube GN rules use pw_toolchain_SCOPE.cc to detect whether to
use GCC or clang warning configuration. At some point,
pw_toolchain_SCOPE.cc changed from just the file name of the compiler
to the full relative path to the compiler, presumably to remove a
PATH dependency.

Unfortunately, that broke the clang vs gcc detection which used
exact string matching against "clang".

This change uses get_path_info(pw_toolchain_SCOPE.cc, "file") to
to extract just the file name.

Change-Id: I73160f63315bfdcc0dc655916ec06e6617311ace
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/119150
Reviewed-by: Wyatt Hepler <cca3bedb37979eacd157a9bceb473582df8aea1e@google.com>
Commit-Queue: Wyatt Hepler <cca3bedb37979eacd157a9bceb473582df8aea1e@google.com>
Pigweed-Auto-Submit: Wyatt Hepler <cca3bedb37979eacd157a9bceb473582df8aea1e@google.com>

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed