Add wrapper to go tool to set the correct CC on cygwin

Go doesn't work as a native cygwin thing, so we need to give it
a mingw cross compiler. Assumes amd64 Go.
bin/go: Check if `go tool pprof` was modified to work with cygwin

It's dumb and hardcodes tool names. On 32-bit cygwin, the regular
objtool doesn't recognize 64-bit binaries, so make pprof use the x86_64
cross compiler's objtool.