TARGET_linux: ensure clang builds can also find libgcc.a & libgcov.a

Summary:
When building globally with clang is fully integrated,
all this cruft should be removed.

The issue: Currently with the wrapper, TARGET_CC *is* clang,
so it is unable to find libgcc.a in a standard build.

The fix: Ensure GCC is always used for finding libgcc.a

Test Plan: verified

Reviewers: randall.rushing, synergy

Reviewed By: synergy

Differential Revision: http://review.codefirex.com/D595

Change-Id: Ic0a883d664631fa018323f188fb53833535eda19