[compiler-rt] Avoid unnecessarily hiding inline visibility [NFC]

Summary:
having fvisibility=hidden obviates the need for
fvisibility-inlines-hidden.

Reviewers: cryptoad, weimingz, mgorny, vsk, compnerd, peter.smith, nikhgupt

Reviewed By: vsk, nikhgupt

Subscribers: dberris, mgorny

Differential Revision: https://reviews.llvm.org/D40269

llvm-svn: 319257