[dev.regabi] runtime: zero X15 in racecall

racecall can be called in ABIInternal context (e.g. raceread
calling racecalladdr calling racecall) without wrapper. racecall
calls C code, which doesn't preserve our special registers. Set
them explicitly in racecall upon returning from C.

Change-Id: Ic990479c1fca6bb8a3b151325c7a89be8331a530
Reviewed-on: https://go-review.googlesource.com/c/go/+/289709
Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>

src/runtime/race_amd64.s

@@ -412,6 +412,9 @@ call:
 	ANDQ	$~15, SP	// alignment for gcc ABI
 	CALL	AX
 	MOVQ	R12, SP
+	// Back to Go world, set special registers.
+	// The g register (R14) is preserved in C.
+	XORPS	X15, X15
 	RET
 
 // C->Go callback thunk that allows to call runtime·racesymbolize from C code.