perf: initialize only native LLVM target

[DaniPopes]

Initialize only the host LLVM target instead of every target built into LLVM. revmc creates target machines for the native triple, so retaining the other target backends is unnecessary. Initialization failures are now propagated as well.

Binary size

The revmc CLI was built against LLVM 22.1.8 component archives with llvm-prefer-static; both outputs were confirmed to have no dynamic LLVM dependency.

Profile	Before	After	Change
profiling	254,582,504 bytes	177,592,928 bytes	-76,989,576 bytes (-30.24%)
maxperf	143,331,920 bytes	76,399,256 bytes	-66,932,664 bytes (-46.70%)

[codspeed-hq]

Merging this PR will not alter performance

✅ 71 untouched benchmarks

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_{Comparing dani/init-native-llvm-target (7840dc8) with main (b5b60c1)}