runtime: convert a darwin/arm64 syscall to libc

Change-Id: I5704a07375fc672ac70c1f4e8df6f4fff760b4bf
Reviewed-on: https://go-review.googlesource.com/118117
Run-TryBot: Elias Naur <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Keith Randall <[email protected]>

Author: Elias Naur

src/runtime/rt0_darwin_arm64.s

@@ -8,8 +8,7 @@
 // supports external linking.
 TEXT _rt0_arm64_darwin(SB),NOSPLIT|NOFRAME,$0
 	MOVD	$42, R0
-	MOVD	$1, R16	// SYS_exit
-	SVC	$0x80
+	BL  libc_exit(SB)
 
 // When linking with -buildmode=c-archive or -buildmode=c-shared,
 // this symbol is called from a global initialization function.