When exporting a function using gccgo, we generate two functions: a Go
function with a leading Cgoexp_ prefix, and a C function that calls the
Go function.  The Go function has a name that can not be represented in
C, so the C code needs a declaration with an __asm__ qualifier giving
the name of the Go function.

Before this CL we put that declaration in the exported header file.
Because code would sometimes #include "_cgo_export.h", we added a macro
definition for the C function giving it the name of the declaration.  We
then added a macro undefine in the actual C code, so that we could
declare the C function we wanted.

This rounadabout process worked OK until we started exporting the header
file for use with -buildmode=c-archive and c-shared.  Doing that caused
the code to see the define and thus call the Go function rather than the
C function.  That often works fine, but the C function calls
_cgo_wait_runtime_init_done before calling the Go function, and that
sometimes matters.  This didn't show up in tests because we don't test
using gccgo.  That is something we should fix, but not now.

Fix that by simplifying the code to declare the C function in the header
file as one would expect, and move the __asm__ declaration to the C
code.

Change-Id: I33547e028152ff98e332630994b4f33285feec32
Reviewed-on: https://go-review.googlesource.com/15043Reviewed-by: Minux Ma <minux@golang.org>