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