Clang warning about ignored stdcall attribute on Cygwin

[ivmai]

Source: master (92abef5)
Host: Win64 (cygwin64)
Compiler: clang version 8.0.1 / x86_64-unknown-windows-cygnus
How to reproduce: clang -O3 -Wall -Wextra -Wpedantic -I include -DGC_THREADS -I ../libatomic_ops/src -c mach_dep.c
Output:

In file included from mach_dep.c:16:
In file included from include/private/gc_priv.h:60:
include/gc/gc.h:2045:21: warning: calling convention '__stdcall__' ignored for this target [-Wignored-attributes]
      GC_API HANDLE WINAPI GC_CreateThread(
                    ^
/usr/include/w32api/minwindef.h:98:16: note: expanded from macro 'WINAPI'
#define WINAPI __stdcall
               ^
<built-in>:345:34: note: expanded from here
#define __stdcall __attribute__((__stdcall__))
                                 ^
In file included from mach_dep.c:16:
In file included from include/private/gc_priv.h:60:
include/gc/gc.h:2052:37: warning: calling convention '__stdcall__' ignored for this target [-Wignored-attributes]
      GC_API DECLSPEC_NORETURN void WINAPI GC_ExitThread(
                                    ^
/usr/include/w32api/minwindef.h:98:16: note: expanded from macro 'WINAPI'
#define WINAPI __stdcall
               ^
<built-in>:345:34: note: expanded from here
#define __stdcall __attribute__((__stdcall__))
                                 ^
In file included from mach_dep.c:16:
In file included from include/private/gc_priv.h:60:
include/gc/gc.h:2066:35: warning: calling convention '__stdcall__' ignored for this target [-Wignored-attributes]
                        unsigned (__stdcall *)(void *),
                                  ^
<built-in>:345:34: note: expanded from here
#define __stdcall __attribute__((__stdcall__))
                                 ^
3 warnings generated.