[compiler-rt] Build all alias in builtin as private external on Darwin

Summary:
For builtin compiler-rt, it is built with visibility hidden by default
to avoid the client exporting symbols from libclang static library. The
compiler option -fvisibility=hidden doesn't work on the aliases in c files
because they are created with inline assembly. On Darwin platform,
thoses aliases are exported by default if they are reference by the client.

Fix the issue by adding ".private_extern" to all the aliases if the
library is built with visibility hidden.

rdar://problem/58960296

Reviewers: dexonsmith, arphaman, delcypher, kledzik

Reviewed By: delcypher

Subscribers: dberris, jkorous, ributzka, #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D73577