cmd/ld: drop gcargs, gclocals symbols from symbol table

Update #6853

Every function now has a gcargs and gclocals symbol
holding associated garbage collection information.
Put them all in the same meta-symbol as the go.func data
and then drop individual entries from symbol table.

Removing gcargs and gclocals reduces the size of a
typical binary by 10%.

LGTM=r
R=r
CC=golang-codereviews
https://golang.org/cl/65870044

Author: rsc

Diff for: src/cmd/ld/symtab.c

@@ -409,5 +409,10 @@ symtab(void)
 			s->hide = 1;
 			s->outer = symgofunc;
 		}
+		if(strstr(s->name, ".gcargs·") != nil || strstr(s->name, ".gclocals·") != nil || strncmp(s->name, "gcargs·", 8) == 0 || strncmp(s->name, "gclocals·", 10) == 0) {
+			s->type = SGOFUNC;
+			s->hide = 1;
+			s->outer = symgofunc;
+		}
 	}
 }