runtime: fix nacl/amd64p32 build

C compiler does not support unnamed fields.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/124870043

src/pkg/runtime/defs_nacl_amd64p32.h

@@ -79,7 +79,7 @@ struct ExcContext
 	union {
 		ExcRegs386	regs;
 		ExcRegsAmd64	regs64;
-	};
+	} regs;
 };
 
 struct ExcPortableContext

src/pkg/runtime/signal_nacl_amd64p32.h

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-#define SIG_REGS(ctxt) (((ExcContext*)(ctxt))->regs64)
+#define SIG_REGS(ctxt) (((ExcContext*)(ctxt))->regs.regs64)
 
 #define SIG_RAX(info, ctxt) (SIG_REGS(ctxt).rax)
 #define SIG_RBX(info, ctxt) (SIG_REGS(ctxt).rbx)