Fix llvm loading when calling through a symlink. (Backport of 87b9278607db44c02c3e6b17342089dae164be49)

    mini-llvm.h: When trying to load mono from ./.libs or ../lib we did not
    resolve the current binary for symlinks. This failed on OSX since
    everyone calls /usr/bin/mono while libmono-llvm was on
    /Library/Frameworks/Mono.framework/Versions/Current/lib.

    This make mono --llvm work on release mono.