MachO: look for nearby tools on all MachO platforms.

This brings "-arch armv7m" (etc) behaviour more in line with what's expected
for developers on OS X, and allows Clang to find an "ld" (for example) in the
same directory instead of using the default /usr/bin/ld.

Unfortunately no test because it relies on the specific place Clang is running
from.

rdar://problem/16427320