Don't export symbols in every binary on linux.

On freebsd this makes sure that symbols are exported on the binaries that need
them. The net result is that we should get symbols in the binaries that need
them on every platform.

On linux x86-64 this reduces the size of the bin directory from 262MB to 250MB.

Patch by Stephen Checkoway.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@178725 91177308-0d34-0410-b5e6-96231b3b80d8
Explicitly add -Wl,--export-all-symbols on mingw/cygwin.

Looks like cmake on windows is not expanding ENABLE_EXPORTS to
-Wl,--export-all-symbols on mingw or cygwin, so add this back.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@178730 91177308-0d34-0410-b5e6-96231b3b80d8