runtime: align C library startup calls on amd64

This makes GOEXPERIMENT=framepointer, GOOS=darwin, and buildmode=carchive coexist.

Change-Id: I9f6fb2f0f06f27df683e5b51f2fa55cd21872453
Reviewed-on: https://go-review.googlesource.com/23454Reviewed-by: Austin Clements <austin@google.com>

src/runtime/rt0_darwin_amd64.s

@@ -12,7 +12,14 @@ TEXT _rt0_amd64_darwin(SB),NOSPLIT,$-8
 
 // When linking with -shared, this symbol is called when the shared library
 // is loaded.
-TEXT _rt0_amd64_darwin_lib(SB),NOSPLIT,$0x48
+TEXT _rt0_amd64_darwin_lib(SB),NOSPLIT,$0x58
+	// Align stack. We don't know whether Go is adding a frame pointer here or not.
+	MOVQ	SP, R8
+	SUBQ	$16, R8
+	ANDQ	$~15, R8
+	XCHGQ	SP, R8
+
+	MOVQ	R8, 0x48(SP)
 	MOVQ	BX, 0x18(SP)
 	MOVQ	BP, 0x20(SP)
 	MOVQ	R12, 0x28(SP)
@@ -51,6 +58,9 @@ restore:
 	MOVQ	0x30(SP), R13
 	MOVQ	0x38(SP), R14
 	MOVQ	0x40(SP), R15
+	
+	MOVQ	0x48(SP), R8
+	MOVQ	R8, SP
 	RET
 
 TEXT _rt0_amd64_darwin_lib_go(SB),NOSPLIT,$0