cmd/cgo: use size_t instead of __SIZE_TYPE__

cmd/cgo: use size_t instead of __SIZE_TYPE__

__SIZE_TYPE__ is a GCC type which has been superseded
by size_t -define in stddef.h- since ISO C99.

cmd/cgo already uses size_t in many places, but still generates several
files using __SIZE_TYPES__, most notably the _cgo_export.h.

This change replaces all __SIZE_TYPES__ occurrences with size_t.

Updates #36233

Change-Id: Id8a99b5d7763caab9333eab9b585e78249a37415
Reviewed-on: https://go-review.googlesource.com/c/go/+/379474
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>