Support LIBRARY_PATH on all Darwin targets.

r197490 changed the behavior of LIBRARY_PATH to try to match GCC's behavior
for cross compilers and make clang work better on "bare metal" targets.
Unfortunately that change is breaking a number of MacPorts projects because
the LIBRARY_PATH environment variable is being ignored when compiling on a
64-bit host for a 32-bit target. Because the host and target architectures
differ, isCrossCompiling returns true. This does not make sense for Darwin,
where multiple architectures are supported natively via "fat" Mach-O slices
and where development is generally done against SDKs regardless. This patch
fixes the problem by overriding isCrossCompiling to return false for Darwin
toolchains.