cmd/dist: generate offsets for P structure

So far no checked-in assembly needs these,
but it matches having them for M and G.
I needed these for some manual testing.

R=golang-dev, dvyukov
CC=golang-dev
https://golang.org/cl/11595043

src/cmd/dist/buildruntime.c

@@ -248,6 +248,8 @@ ok:
 				aggr = "g";
 			else if(streq(fields.p[1], "M"))
 				aggr = "m";
+			else if(streq(fields.p[1], "P"))
+				aggr = "p";
 			else if(streq(fields.p[1], "Gobuf"))
 				aggr = "gobuf";
 			else if(streq(fields.p[1], "WinCall"))