Explicitly allow read-only relocations when building on OS X in 32 bit mode

This fixes building libwels.dylib, since the assembly code
isn't position independent.
Merge pull request #314 from mstorsjo/fix-osx-32-bit

Explicitly allow read-only relocations when building on OS X in 32 bit mode
Merge remote-tracking branch 'upstream/master' into travis.osx