stand: -fno-common fixes for !x86 loaders

- beriloader: archsw is declared extern and defined elsewhere
- ofwloader: ofw_elf{,64} are defined in elf_freebsd.c and
  ppc64_elf_freebsd.c respectively
- ubldr: syscall_ptr is defined in start.S for whichever ubldr platform is
  building

-fno-common will become the default in GCC10/LLVM11.

MFC after:	3 days

stand/mips/beri/loader/main.c

@@ -59,8 +59,6 @@ struct devsw *devsw[] = {
 	NULL
 };
 
-struct arch_switch archsw;
-
 struct file_format *file_formats[] = {
 	&beri_elf,
 	NULL

stand/powerpc/ofw/conf.c

@@ -97,8 +97,8 @@ struct netif_driver *netif_drivers[] = {
  * rather than reading the file go first.
  */
 
-struct file_format ofw_elf;
-struct file_format ofw_elf64;
+extern struct file_format ofw_elf;
+extern struct file_format ofw_elf64;
 
 struct file_format *file_formats[] = {
     &ofw_elf,

stand/uboot/lib/glue.h

@@ -56,7 +56,7 @@
 #endif
 
 int syscall(int, int *, ...);
-void *syscall_ptr;
+extern void *syscall_ptr;
 
 int api_parse_cmdline_sig(int argc, char **argv, struct api_signature **sig);
 int api_search_sig(struct api_signature **sig);