[PowerPC] Fix ELF interpreter name for powerpc64le-linux

On PowerPC LE the system uses the /lib64/ld64.so.2 dynamic linker name
instead of /lib64/ld64.so.1 (to indicate the ELFv2 ABI version).

This fixes the clang driver to pass the appropriate -dynamic-linker
setting, and adds some more tests to linux-ld.c.