runtime: save callee-saved regs in darwin-amd64 library init

We're only getting away with it today by luck.

Change-Id: I24d1cceee4d20c5181ca64fceda152e875f6ad81
Reviewed-on: https://go-review.googlesource.com/18440
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

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,$40
+TEXT _rt0_amd64_darwin_lib(SB),NOSPLIT,$0x48
+	MOVQ	BX, 0x18(SP)
+	MOVQ	BP, 0x20(SP)
+	MOVQ	R12, 0x28(SP)
+	MOVQ	R13, 0x30(SP)
+	MOVQ	R14, 0x38(SP)
+	MOVQ	R15, 0x40(SP)
+
 	MOVQ	DI, _rt0_amd64_darwin_lib_argc<>(SB)
 	MOVQ	SI, _rt0_amd64_darwin_lib_argv<>(SB)
 
@@ -23,7 +30,8 @@ TEXT _rt0_amd64_darwin_lib(SB),NOSPLIT,$40
 	MOVQ	$_rt0_amd64_darwin_lib_go(SB), DI
 	MOVQ	$0, SI
 	CALL	AX
-	RET
+	JMP	restore
+
 nocgo:
 	MOVQ	$8388608, 0(SP)                    // stacksize
 	MOVQ	$_rt0_amd64_darwin_lib_go(SB), AX
@@ -31,6 +39,14 @@ nocgo:
 	MOVQ	$0, 16(SP)                         // fnarg
 	MOVQ	$runtime·newosproc0(SB), AX
 	CALL	AX
+
+restore:
+	MOVQ	0x18(SP), BX
+	MOVQ	0x20(SP), BP
+	MOVQ	0x28(SP), R12
+	MOVQ	0x30(SP), R13
+	MOVQ	0x38(SP), R14
+	MOVQ	0x40(SP), R15
 	RET
 
 TEXT _rt0_amd64_darwin_lib_go(SB),NOSPLIT,$0