Driver: Make profiling flags work with -nostdlib on Darwin

Compiler-rt's Profiling library isn't part of the stdlib, so -nostdlib
shouldn't prevent it from being linked. This makes Darwin behave like
other toolchains, and link in the profile runtime irrespective of
-nostdlib, since the resulting program can't be run unless you link
this.

I've also added a test to show that other toolchains already behave
like this.

llvm-svn: 237074