fix(setjmp): restore caller rbx in sigsetjmp mask path

src/setjmp/x86_64/sigsetjmp.s

@@ -28,6 +28,12 @@
  *
  * The mask slot is written through &env[11]; on success the flag is set and
  * 0 is returned in eax, exactly like plain setjmp.
+ *
+ * rbx carries the env base across the syscall (rdi/rsi/rdx/r10 hold the
+ * syscall arguments, rax the number, rcx/r11 are clobbered by the kernel).
+ * rbx is callee-saved: its original value was stored in env[0] above, so it
+ * is reloaded from there BEFORE the ret, leaving every callee-saved register
+ * intact on the ordinary return path as the SysV AMD64 ABI requires.
  */
 
 .global __sigsetjmp
@@ -51,7 +57,7 @@ sigsetjmp:
 	fnstcw 72(%rdi)
 	test %esi,%esi
 	jz 1f
-	mov %rdi,%rbx        /* env base (rbx is restored from the buffer later) */
+	mov %rdi,%rbx        /* env base (scratch; caller rbx is in env[0]) */
 	mov $14,%eax         /* SYS_rt_sigprocmask */
 	xor %edi,%edi        /* how = SIG_BLOCK(0) */
 	xor %esi,%esi        /* set = NULL (query only) */
@@ -59,6 +65,7 @@ sigsetjmp:
 	mov $8,%r10d         /* sigsetsize (x86_64 sigset_t = one word) */
 	syscall
 	movq $1,80(%rbx)     /* mask saved: set the flag */
+	mov 0(%rbx),%rbx     /* restore the caller's rbx before returning */
 1:
 	xor %eax,%eax
 	ret