crunchide: Restore local EM_AARCH64 constant for bootstrapping

Most of the EM_* constants are available in all supported host branches,
but EM_AARCH64 was added relatively recently. Add it back to fix
building HEAD on 10.x.

Noticed by:	adrian, jmallett

usr.sbin/crunch/crunchide/exec_elf32.c

@@ -178,6 +178,9 @@ ELFNAMEEND(check)(int fd, const char *fn)
 	switch (xe16toh(eh.e_machine)) {
 	case EM_386: break;
 	case EM_ALPHA: break;
+#ifndef EM_AARCH64
+#define	EM_AARCH64	183
+#endif
 	case EM_AARCH64: break;
 	case EM_ARM: break;
 	case EM_MIPS: break;