cmd/dist: support building pkg/runtime/*.

Essentialy for running tests without a working cmd/go.
While we're at it, also fix a typo.

LGTM=rsc
R=golang-codereviews, rsc
CC=golang-codereviews
https://golang.org/cl/70640043

src/cmd/dist/build.c

@@ -808,7 +808,8 @@ install(char *dir)
 	files.len = n;
 
 	for(i=0; i<nelem(deptab); i++) {
-		if(hasprefix(dir, deptab[i].prefix)) {
+		if(streq(dir, deptab[i].prefix) ||
+		   (hassuffix(deptab[i].prefix, "/") && hasprefix(dir, deptab[i].prefix))) {
 			for(j=0; (p=deptab[i].dep[j])!=nil; j++) {
 				breset(&b1);
 				bwritestr(&b1, p);

src/cmd/dist/buildruntime.c

@@ -346,7 +346,7 @@ ok:
 	vfree(&fields);
 }
 
-// mkzsys writes zsys_$GOOS_$GOARCH.h,
+// mkzsys writes zsys_$GOOS_$GOARCH.s,
 // which contains arch or os specific asm code.
 // 
 void