[clang] [PowerPC] Add an alias for -mregnames so that full register names used in assembly. (PR #70255)

[Fangrui Song via cfe-commits]

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+// REQUIRES: powerpc-registered-target
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MaskRay wrote:

Tests at the wrong layer: https://maskray.me/blog/2021-08-08-toolchain-testing#the-test-checks-at-the-wrong-layer

This should just test that clang passes -target-feature. It's unnecessary to enumerate every combination.

Also, use `--target=` for new tests. `-target ` has been deprecated since 3.4.

https://github.com/llvm/llvm-project/pull/70255