[builtins] Use %% as the separator for aarch64-*-darwin and ; for powerpc{32,64}

`;` is the default comment marker, which is also used by powerpc*-*-elf target triples.
`@` is the comment marker of powerpc*-*-darwin but the Darwin support has been deleted for PowerPC (D72063).
`%%` is the statement separator used by aarch64-*-darwin (see AArch64MCAsmInfoDarwin, it uses `;` as the comment marker, which is different from most other targets)

Reviewed By: tambre

Differential Revision: https://reviews.llvm.org/D93378