gcc translates -gz=zlib to --compress-debug-options=zlib for both assembler and linker
but clang only does this for assembler.
The linker needs --compress-debug-options=zlib option to compress the debug sections
in the generated executable or shared library.
Due to this bug, -gz=zlib has no effect on the generated executable or shared library.
This patch fixes that.
If the user sets CLANG_DEFAULT_LINKER to a string not containing "ld", this line will not match. Matching a different substring on the linker command line can be more robust, e.g. "-m" "elf_x86_64"