[AArch64][test] Fix test failing on unknown options (#139696)
Fixes buildbot failure
https://lab.llvm.org/buildbot/#/builders/16/builds/18873 originating
from #138448. Normally ignored silently but fails on higher error
levels.
Buildbot errors:
```
/b/1/llvm-clang-x86_64-expensive-checks-debian/build/bin/llc < /b/1/llvm-clang-x86_64-expensive-checks-debian/llvm-project/llvm/test/CodeGen/AArch64/reserveXreg.ll -mtriple=aarch64-unknown-linux-gnu | /b/1/llvm-clang-x86_64-expensive-checks-debian/build/bin/FileCheck /b/1/llvm-clang-x86_64-expensive-checks-debian/llvm-project/llvm/test/CodeGen/AArch64/reserveXreg.ll # RUN: at line 6
+ /b/1/llvm-clang-x86_64-expensive-checks-debian/build/bin/FileCheck /b/1/llvm-clang-x86_64-expensive-checks-debian/llvm-project/llvm/test/CodeGen/AArch64/reserveXreg.ll
+ /b/1/llvm-clang-x86_64-expensive-checks-debian/build/bin/llc -mtriple=aarch64-unknown-linux-gnu
'+reserve-x8' is not a recognized feature for this target (ignoring feature)
'+reserve-x8' is not a recognized feature for this target (ignoring feature)
'+reserve-x16' is not a recognized feature for this target (ignoring feature)
'+reserve-x16' is not a recognized feature for this target (ignoring feature)
'+reserve-x17' is not a recognized feature for this target (ignoring feature)
'+reserve-x17' is not a recognized feature for this target (ignoring feature)
```