Avoid ar_to_bc.sh script in symbolizer build; add -ignore-non-bitcode flag to llvm-link

Remove the `ar_to_bc.sh` helper script in the compiler-rt symbolizer
build. Instead use `llvm-link` directly as D81109 introduced the
capability to read archives. I just needed to add a new flag
`-ignore-non-bitcode` to avoid `llvm-link` aborting when it found a
non-bitcode file in the archive.

Differential Revision: https://reviews.llvm.org/D128729