[dev.regabi] runtime/cgo: call setg_gcc in crosscall_amd64
Currently, when using cgo, the g pointer is set via a separate
call to setg_gcc or with inline assembly in threadentry. This CL
changes it to call setg_gcc in crosscall_amd64, like other g-
register platforms. When we have an actual g register on AMD64,
we'll need to set the register immediately before calling into
Go.
Change-Id: Ib1171e05cd0dabba3b7d12e072084d141051cf3d
Reviewed-on: https://go-review.googlesource.com/c/go/+/289192
Trust: Cherry Zhang <cherryyz@google.com> Reviewed-by: Ian Lance Taylor <iant@golang.org> Reviewed-by: Than McIntosh <thanm@google.com>