Optimize umask_get (make it a leaf function)

src/core.asm

@@ -199,10 +199,12 @@ umask_set:
 ;	rdi*	syscall arg
 ;	rdx*	Stores umask for restoring+returning umask
 umask_get:
+	mov	rax, NR_umask
 	xor	rdi, rdi
-	call	umask_set
+	syscall
 	mov	rdx, rax
 	mov	rdi, rax
-	call	umask_set
+	mov	rax, NR_umask
+	syscall
 	mov	rax, rdx
 	ret