crt: Restore arch checks in __signbit.

This partially reverts dbb60ad07c29830, which accidentally removed __x86_64__
check. The change is not needed for ARM64EC, which defined __x86_64__ too.

Signed-off-by: Jacek Caban <jacek@codeweavers.com>

mingw-w64-crt/math/signbit.c

diff --git a/mingw-w64-crt/math/signbit.c b/mingw-w64-crt/math/signbit.c
index b123b51..a97d8b9 100644
--- a/mingw-w64-crt/math/signbit.c
+++ b/mingw-w64-crt/math/signbit.c
@@ -16,7 +16,7 @@
 
 int __signbit (double x)
 {
-#if __SIZEOF_LONG_DOUBLE__ == __SIZEOF_DOUBLE__
+#if defined(__x86_64__) || defined(_AMD64_) || defined(__arm__) || defined(_ARM_) || defined(__aarch64__) || defined(_ARM64_)
     __mingw_dbl_type_t hlp;
     
     hlp.x = x;