runtime: cross-reference _func type better

It takes me several minutes every time I want to find where the linker
writes out the _func structures. Add some comments to make this
easier.

Change-Id: Ic75ce2786ca4b25726babe3c4fe9cd30c85c34e2
Reviewed-on: https://go-review.googlesource.com/34390
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: aclements

src/cmd/link/internal/ld/pcln.go

@@ -248,6 +248,9 @@ func (ctxt *Link) pclntab() {
 		setaddr(ctxt, ftab, 8+int64(SysArch.PtrSize)+int64(nfunc)*2*int64(SysArch.PtrSize), s)
 		setuintxx(ctxt, ftab, 8+int64(SysArch.PtrSize)+int64(nfunc)*2*int64(SysArch.PtrSize)+int64(SysArch.PtrSize), uint64(funcstart), int64(SysArch.PtrSize))
 
+		// Write runtime._func. Keep in sync with ../../../../runtime/runtime2.go:/_func
+		// and package debug/gosym.
+
 		// fixed size of struct, checked below
 		off := funcstart
 

src/runtime/runtime2.go

@@ -615,7 +615,7 @@ const (
 
 // Layout of in-memory per-function information prepared by linker
 // See https://golang.org/s/go12symtab.
-// Keep in sync with linker
+// Keep in sync with linker (../cmd/link/internal/ld/pcln.go:/pclntab)
 // and with package debug/gosym and with symtab.go in package runtime.
 type _func struct {
 	entry   uintptr // start pc