[libc++abi] Do not define -Dcxxabi_shared_EXPORTS when building libc++abi

CMake sets adds that definition automatically, but we don't need
or use it.

llvm-svn: 373940
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: d03068c3e1fbc8b8aa24af8e2a806fafa8a92e26
1 file changed