The use of an empty import "C" to trigger cgo in runtime/race
serves two purposes:
1. Cause the runtime to use the C library to create system threads,
because the race syso implementation expects things like
thread-local storage to work correctly.
2. Derive the right set of //go:cgo_import_dynamic comments
to pass to the Go linker, so that it doesn't diagnose them as
undefined references.
On macOS, (1) is unnecessary because using the C library
(via DLL calls) is the only way the runtime ever creates threads.
We can accomplish (2) by writing those comments ourselves.
Having done that in this CL, cgo is no longer needed to run
the race detector on macOS, which means that having a
pre-compiled set of .a files is no longer necessary,
nor is having Xcode for use with cgo when rebuilding those .a files.