Cygwin: signal: Clear direction flag in sigdeleyed

x86_64 ABI requires the direction flag in CPU flags register cleared.
https://learn.microsoft.com/en-us/cpp/build/x64-software-conventions
However, currently that flag is not maintained in signal handler.
Therefore, if the signal handler is called when that flag is set, it
destroys the data and may crash if rep instruction is used in the
signal handler. With this patch, the direction flag is cleared in
sigdelayed() by adding cld instruction.

Backported-from: d71aecc (Cygwin: signal: Clear direction flag in sigdeleyed, 2025-03-24)
Addresses: https://cygwin.com/pipermail/cygwin/2025-March/257704.html
Fixes: 1fd5e00 ("import winsup-2000-02-17 snapshot")
Reported-by: Christian Franke <[email protected]>
Reviewed-by: Corinna Vischen <[email protected]>
Signed-off-by: Takashi Yano <[email protected]>
(cherry picked from commit 9f4cec47df66a913256912c358afe5e597dec25c)
Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

winsup/cygwin/release/3.6.1

@@ -3,3 +3,6 @@ Fixes:
 
 - Console mode is really restored to the previous mode.
   Addresses: https://github.com/msys2/msys2-runtime/issues/268
+
+- Clear direction flag in sigdeleyed before calling signal handler.
+  Addresses: https://cygwin.com/pipermail/cygwin/2025-March/257704.html

winsup/cygwin/scripts/gendef

@@ -179,6 +179,7 @@ sigdelayed:
 	movq	%rsp,%rbp
 	pushf
 	.seh_pushreg %rax			# fake, there's no .seh_pushreg for the flags
+	cld					# x86_64 ABI requires direction flag cleared
 	# stack is aligned or unaligned on entry!
 	# make sure it is aligned from here on
 	# We could be called from an interrupted thread which doesn't know