cmd/5l, cmd/8g: fix build failures

REG_R0 etc are defined in <ucontext.h> on ARM systems.
Possible use of uninitialized n in 8g/reg.c.

Change-Id: I6e8ce83a6515ca2b779ed8a344a25432db629cc2
Reviewed-on: https://go-review.googlesource.com/3578Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/5l/5.out.h

@@ -40,6 +40,24 @@ enum
 #define	REGARG		-1
 /*c2go enum { REGARG = -1 }; */
 
+// avoid conflict with ucontext.h. sigh.
+#define REG_R0 GO_REG_R0
+#define REG_R1 GO_REG_R1
+#define REG_R2 GO_REG_R2
+#define REG_R3 GO_REG_R3
+#define REG_R4 GO_REG_R4
+#define REG_R5 GO_REG_R5
+#define REG_R6 GO_REG_R6
+#define REG_R7 GO_REG_R7
+#define REG_R8 GO_REG_R8
+#define REG_R9 GO_REG_R9
+#define REG_R10 GO_REG_R10
+#define REG_R11 GO_REG_R11
+#define REG_R12 GO_REG_R12
+#define REG_R13 GO_REG_R13
+#define REG_R14 GO_REG_R14
+#define REG_R15 GO_REG_R15
+
 enum
 {
 	REG_R0 = 32, // must be 16-aligned

src/cmd/8g/reg.c

@@ -615,6 +615,8 @@ mkvar(Reg *r, Adr *a)
 
 	case TYPE_MEM:
 		switch(a->name) {
+		default:
+			goto none;
 		case NAME_EXTERN:
 		case NAME_STATIC:
 		case NAME_PARAM: