Fix -Wunused-command-line-argument warnings when --force_pic is passed on Mac

[jamiesnape]

For clang on macOS, there is no -pie command line argument, so you need -Wl,-pie to pass the flag to the linker and avoid the following warning:

INFO: From Linking <executable>:
clang: warning: argument unused during compilation: '-pie' [-Wunused-command-line-argument]

[jamiesnape]

Resolved merge conflicts. Can this at least be approved for presubmit testing even if review is delayed?

[lberki]

Sorry for the late reply!

@oquenchil , what do you think? It's a bit more special-casing, but it doesn't make the situation much worse... I'd say go ahead.

[hlopko]

LGTM, thanks! I'll import this now.