cleanup: use full `strip` cli args in cg_ssa

[jieyouxu]

Suggestion [DISCARD-ALL 1/2]: can we spell out the full flag for this as well? llvm-objcopy docs say that -x is

--discard-all, -x

Remove most local symbols from the output. Different file formats may limit this to a subset of the local symbols. For example, file and section symbols in ELF objects will not be discarded. Additionally, remove all debug sections.

This happens to align with llvm-strip, unlike -S or no-flag.

• https://llvm.org/docs/CommandGuide/llvm-strip.html#cmdoption-llvm-strip-discard-all
• https://llvm.org/docs/CommandGuide/llvm-objcopy.html#cmdoption-llvm-objcopy-discard-all

Originally posted by @jieyouxu in #135034 (comment)

[jieyouxu]

Note: this depends on what the concrete strip-like command cli scheme is for a given platform in cg_ssa/back/link.rs.