Add the new `-no-builtin' flag. This flag is meant to mimic the GCC

`-fno-builtin' flag. Currently, it's used to replace "memset" with "_bzero"
instead of "__bzero" on Darwin10+. This arguably violates the meaning of this
flag, but is currently sufficient. The meaning of this flag should become more
specific over time.

llvm-svn: 56885