runtime/cgo: merge bodies of cgo_sys_thread_start on windows
The bodies of cgo_sys_thread_start (and x_cgo_sys_thread_create) are
nearly identical on all of the windows ports.
Create a single _cgo_beginthread implementation that contains the body
and is used on all ports. This will reduce churn in an upcoming CL to
add retry logic.
We could theoretically have a single implementation of
_cgo_sys_thread_start shared by all ports, but I keep them separate for
ease of searching. Right now every single port implements this function
in their gcc_GOOS_GOARCH.c file, so it is nice to keep this symmetry.
_cgo_dummy_export must move out of libcgo_windows.h because it is a
definition and the inclusion of libcgo_windows.h in multiple files
creates duplicate definitions.
For #52572.
Change-Id: I9fa22009389349c754210274c7db2631b061f9c7
Reviewed-on: https://go-review.googlesource.com/c/go/+/410354
Run-TryBot: Michael Pratt <mpratt@google.com> Reviewed-by: Michael Knyszek <mknyszek@google.com> Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>