setjmp: fix x86-64 longjmp argument adjustment

longjmp 'val' argument is an int, but the assembly is referencing 64-bit
registers as if the argument was a long, or the caller was responsible
for extending the argument. Though the psABI is not clear on this, the
interpretation in GCC is that high bits may be arbitrary and the callee
is responsible for sign/zero-extending the value as needed (likewise for
return values: callers must anticipate that high bits may be garbage).

Therefore testing %rax is a functional bug: setjmp would wrongly return
zero if longjmp was called with val==0, but high bits of %rsi happened
to be non-zero.

Rewrite the prologue to refer to 32-bit registers. In passing, change
'test' to use %rsi, as there's no advantage to using %rax and the new
form is cheaper on processors that do not perform move elimination.

src/setjmp/x32/longjmp.s

@@ -5,10 +5,10 @@
 .type longjmp,@function
 _longjmp:
 longjmp:
-	mov %rsi,%rax           /* val will be longjmp return */
-	test %rax,%rax
+	mov %esi,%eax           /* val will be longjmp return */
+	test %esi,%esi
 	jnz 1f
-	inc %rax                /* if val==0, val=1 per longjmp semantics */
+	inc %eax                /* if val==0, val=1 per longjmp semantics */
 1:
 	mov (%rdi),%rbx         /* rdi is the jmp_buf, restore regs from it */
 	mov 8(%rdi),%rbp

src/setjmp/x86_64/longjmp.s

@@ -5,10 +5,10 @@
 .type longjmp,@function
 _longjmp:
 longjmp:
-	mov %rsi,%rax           /* val will be longjmp return */
-	test %rax,%rax
+	mov %esi,%eax           /* val will be longjmp return */
+	test %esi,%esi
 	jnz 1f
-	inc %rax                /* if val==0, val=1 per longjmp semantics */
+	inc %eax                /* if val==0, val=1 per longjmp semantics */
 1:
 	mov (%rdi),%rbx         /* rdi is the jmp_buf, restore regs from it */
 	mov 8(%rdi),%rbp