Use OS rather than compiler specific flag the same way that
__FreeBSD__, __APPLE__, __OpenBSD__, and __linux__ are used.

_WIN32 is defined by GCC (and others) on windows for Win32
and Win64 applications. _WIN32 is set by default for several
other windows based compilers: DMC, MSVC, Intel, Watcom, LCC.

Although the change is for consistency, it allows the Go tools
to be compiled with non-Mingw GCC distributions and non-GCC
compilers that support the GCC extensions.

R=rsc, brainman, vcc
CC=golang-dev
https://golang.org/cl/2168043