Fix definition of _setjmp

The #pragma weak does not work, the symbol is not defined and when
already defined, it is not made weak. Moreover, it does not make the
symbol global so we need to declare it with a FUNCTION.

If we want the symbol to be weak, then we should use .weak to do so. The
_longjmp symbol should also be changed to match, in that case.

Fixes #12912.

src/system/libroot/posix/arch/arm/sigsetjmp.S

@@ -19,6 +19,7 @@ FUNCTION_END(__sigsetjmp)
 
 /* int setjmp(jmp_buf buffer) */
 FUNCTION(setjmp):
+FUNCTION(_setjmp):
 	stmia	r0, {r0-r14}
 	mrs	r1, cpsr
 	str	r1, [r0, #64]

src/system/libroot/posix/arch/m68k/sigsetjmp.S

@@ -33,6 +33,7 @@ FUNCTION_END(sigsetjmp)
 
 /* int setjmp(jmp_buf buffer) */
 FUNCTION(setjmp):
+FUNCTION(_setjmp):
 	move.l	(%a7)+,%a0
 	move.l	(%a7)+,%d0
 	clr.l	-(%a7)		/* push 0 as 2nd arg */

src/system/libroot/posix/arch/ppc/sigsetjmp.S

@@ -44,6 +44,7 @@ FUNCTION(sigsetjmp):
 
 /* int setjmp(jmp_buf buffer) */
 FUNCTION(setjmp):
+FUNCTION(_setjmp):
 	// call __sigsetjmp with saveMask = 0
 	addi	%r4, 0, 0
 	b		__sigsetjmp

src/system/libroot/posix/arch/x86/sigsetjmp.S

@@ -43,6 +43,7 @@ FUNCTION_END(sigsetjmp)
 
 /* int setjmp(jmp_buf buffer) */
 FUNCTION(setjmp):
+FUNCTION(_setjmp):
 	// prepare %edx, %ecx, and %eax for sigsetjmp
 	mov		0(%esp), %edx
 	lea		4(%esp), %ecx

src/system/libroot/posix/arch/x86_64/sigsetjmp.S

@@ -30,6 +30,7 @@ FUNCTION_END(setjmp)
 
 /* int setjmp(jmp_buf buffer) */
 FUNCTION(setjmp):
+FUNCTION(_setjmp):
 	// Jump to sigsetjmp with a zero saveMask.
 	xorl	%esi, %esi
 	jmp		sigsetjmp@PLT