runtime/internal/startlinetest: work around shared buildmode linking issue

The runtime/internal/startlinetest package contains a call to a
function defined in runtime_test. Generally this is fine as this
package is only linked in for runtime_test. Except that for "go
install -buildmode=shared std", which include all packages in std,
including this test-only internal package. In this mode, the
caller is included in the linking but the callee is not, causing
linking error. Work around it by calling
runtime_test.callerStartLine via a function pointer. The function
pointer is only set in runtime_test. In the shared std build, the
function pointer will not be set, and this is fine.

Fixes #57334.

Change-Id: I7d871c50ce6599c6ea2802cf6e14bb749deab220
Reviewed-on: https://go-review.googlesource.com/c/go/+/458696
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>

src/runtime/internal/startlinetest/func_amd64.go

@@ -8,3 +8,6 @@ package startlinetest
 // Defined in func_amd64.s, this is a trivial assembly function that calls
 // runtime_test.callerStartLine.
 func AsmFunc() int
+
+// Provided by runtime_test.
+var CallerStartLine func(bool) int

src/runtime/internal/startlinetest/func_amd64.s

@@ -23,5 +23,6 @@
 TEXT	·AsmFunc<ABIInternal>(SB),NOSPLIT,$8-0
 	NO_LOCAL_POINTERS
 	MOVQ	$0, AX // wantInlined
-	CALL    runtime_test·callerStartLine<ABIInternal>(SB)
+	MOVQ	·CallerStartLine(SB), DX
+	CALL	(DX)
 	RET

src/runtime/start_line_amd64_test.go

@@ -13,6 +13,8 @@ import (
 // is only tested on amd64 to avoid the need for a proliferation of per-arch
 // copies of this function.
 func TestStartLineAsm(t *testing.T) {
+	startlinetest.CallerStartLine = callerStartLine
+
 	const wantLine = 23
 	got := startlinetest.AsmFunc()
 	if got != wantLine {