libgo: break dependence on libgcc unwind-pe.h

libgo: break dependence on libgcc unwind-pe.h

    The C portion of the Go runtime includes the header "unwind-pe.h" from
    libgcc, which contains some constants and a few small routines for
    decoding pointer values within unwind info. This patch gets rid of
    that include and instead adds a re-implementation of that
    functionality in the single file that uses it. The intent is to allow
    the C runtime portion of libgo to be built without a companion GCC
    installation.

    Reviewed-on: https://go-review.googlesource.com/90235

From-SVN: r259861