[Hexagon][test] Fix some tests on linux-musl

[Hexagon][test] Fix some tests on linux-musl

-march=hexagon uses the default target triple and changes the arch part of
hexagon. On linux-musl, this essentially becomes hexagon-unknown-linux-musl
which has different code generation. Use -mtriple instead.

Link: https://github.com/llvm/llvm-project/issues/48936