[compiler-rt] Fix library suffixes on windows

Summary:
Make sure that windows libraries contain the suffixes expected by clang.
This is expecially important when compiling the windows builtins with
clang, as cmake will use .a as the suffix by default.

Reviewers: beanz, compnerd

Subscribers: llvm-commits, dberris

Differential Revision: https://reviews.llvm.org/D24046

llvm-svn: 280854