[compiler-rt][libunwind][runtimes] Recategorize `llvm_check_linker_flag` langs

Done according to @phosek's comments in D117537, but not done then to
separate pure refactor (that) from possible behavior change (this).

Wasn't working before, but I think that was due to an issue of mismatched variable names fixed in D110005.

Reviewed By: phosek, #libunwind, #libc_abi

Differential Revision: https://reviews.llvm.org/D117833