cmd/5g, cmd/6g, cmd/8g: fix comment

Keeping the string "compactframe" because that's what
I always search for to find this code. But point to the real place too.

TBR=iant
CC=golang-dev
https://golang.org/cl/10676047

src/cmd/5g/ggen.c

@@ -39,7 +39,7 @@ markautoused(Prog* p)
 	}
 }
 
-// Fixup instructions after compactframe has moved all autos around.
+// Fixup instructions after allocauto (formerly compactframe) has moved all autos around.
 void
 fixautoused(Prog* p)
 {

src/cmd/6g/ggen.c

@@ -36,7 +36,7 @@ markautoused(Prog* p)
 	}
 }
 
-// Fixup instructions after compactframe has moved all autos around.
+// Fixup instructions after allocauto (formerly compactframe) has moved all autos around.
 void
 fixautoused(Prog *p)
 {

src/cmd/8g/ggen.c

@@ -38,7 +38,7 @@ markautoused(Prog* p)
 	}
 }
 
-// Fixup instructions after compactframe has moved all autos around.
+// Fixup instructions after allocauto (formerly compactframe) has moved all autos around.
 void
 fixautoused(Prog* p)
 {