[mach-o] Support -F and -framework options in darwin driver

Darwin has a packaging mechanism for shared libraries and headers called
frameworks.  A directory Foo.framework contains a shared library binary file
"Foo" and a subdirectory "Headers". Most OS frameworks are all in one
directory /System/Library/Frameworks/.  As a linking convenience, the linker
option "-framework Foo" means search the framework directories specified
with -F (analogous to -L) looking for a shared library Foo.framework/Foo.