declare __syscall_ret as hidden in vfork asm

without this, it's plausible that assembler or linker could complain
about an unsatisfiable relocation.

src/process/i386/vfork.s

@@ -9,6 +9,7 @@ vfork:
 	int $128
 	push %edx
 	push %eax
+	.hidden __syscall_ret
 	call __syscall_ret
 	pop %edx
 	ret

src/process/s390x/vfork.s

@@ -5,4 +5,5 @@
 __vfork:
 vfork:
 	svc 190
+	.hidden __syscall_ret
 	jg  __syscall_ret

src/process/x32/vfork.s

@@ -9,4 +9,5 @@ vfork:
 	syscall
 	push %rdx
 	mov %rax,%rdi
+	.hidden __syscall_ret
 	jmp __syscall_ret

src/process/x86_64/vfork.s

@@ -9,4 +9,5 @@ vfork:
 	syscall
 	push %rdx
 	mov %rax,%rdi
+	.hidden __syscall_ret
 	jmp __syscall_ret