cmd/gc: don't generate zillions of linehists for wrapper functions

This is a workaround - the code should be better than this - but the
fix avoids generating large numbers of linehist entries for the wrapper
functions that enable interface conversions. There can be many of
them, they all happen at the end of compilation, and they can all
share a linehist entry.
Avoids bad n^2 behavior in liblink.
Test case in issue 8135 goes from 64 seconds to 2.5 seconds (still bad
but not intolerable).

Fixes #8135.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/104840043

src/cmd/gc/subr.c

@@ -2493,6 +2493,7 @@ genwrapper(Type *rcvr, Type *method, Sym *newnam, int iface)
 	Type *tpad, *methodrcvr;
 	int isddd;
 	Val v;
+	static int linehistdone = 0;
 
 	if(0 && debug['r'])
 		print("genwrapper rcvrtype=%T method=%T newnam=%S\n",
@@ -2500,7 +2501,11 @@ genwrapper(Type *rcvr, Type *method, Sym *newnam, int iface)
 
 	lexlineno++;
 	lineno = lexlineno;
-	linehist("<autogenerated>", 0, 0);
+	if (linehistdone == 0) {
+		// All the wrappers can share the same linehist entry.
+		linehist("<autogenerated>", 0, 0);
+		linehistdone = 1;
+	}
 
 	dclcontext = PEXTERN;
 	markdcl();