build: fix various 'set and not used' for Plan 9

R=dave, minux.ma, rsc
CC=golang-dev
https://golang.org/cl/6501134

src/cmd/8g/reg.c

@@ -147,7 +147,6 @@ regopt(Prog *firstp)
 		return;
 	}
 
-	r1 = R;
 	firstr = R;
 	lastr = R;
 	

src/cmd/gc/closure.c

@@ -252,6 +252,7 @@ walkclosure(Node *func, NodeList **init)
 void
 walkcallclosure(Node *n, NodeList **init)
 {
+	USED(init);
 	if (n->op != OCALLFUNC || n->left->op != OCLOSURE) {
 		dump("walkcallclosure", n);
 		fatal("abuse of walkcallclosure");

src/cmd/gc/esc.c

@@ -949,6 +949,7 @@ esctag(EscState *e, Node *func)
 	Node *savefn;
 	NodeList *ll;
 	
+	USED(e);
 	func->esc = EscFuncTagged;
 	
 	// External functions must be assumed unsafe.

src/cmd/gc/mparith1.c

@@ -288,7 +288,7 @@ mpatoflt(Mpflt *a, char *as)
 	s = as;
 	base = -1;
 	while(base == -1) {
-		switch(c = *s++) {
+		switch(*s++) {
 		case '-':
 		case '+':
 			break;

src/cmd/gc/reflect.c

@@ -471,6 +471,7 @@ typestruct(Type *t)
 	// Otherwise it will use a nil type word but still be usable
 	// by package runtime (because we always use the memory
 	// after the interface value, not the interface value itself).
+	USED(t);
 	return pkglookup("*reflect.commonType", weaktypepkg);
 }
 

src/cmd/ld/go.c

@@ -579,6 +579,7 @@ loaddynlinker(char *file, char *pkg, char *p, int n)
 	char *pend, *next, *dynlinker, *p0;
 
 	USED(file);
+	USED(pkg);
 	pend = p + n;
 	for(; p<pend; p=next) {
 		next = strchr(p, '\n');

src/pkg/runtime/thread_plan9.c

@@ -356,5 +356,6 @@ static int8 badsignal[] = "runtime: signal received on thread not created by Go.
 void
 runtime·badsignal(int32 sig)
 {
+	USED(sig);
 	runtime·pwrite(2, badsignal, sizeof badsignal - 1, -1LL);
 }