Make arm64 32-bit mode compile with COMPAT_43

The COMPAT_43 option isn't quite like the other compat options, and arm64 makes
attempts to support it in 64-bit mode. In 32-bit compat mode, however, two
syscall implementations that COMPAT_FREEBSD32 assumes will be there are
missing. Provide implementations for these: ofreebsd32_sigreturn (which we'll
never encounter, so implement it as nosys as is done in kern_sig.c) and
ofreebsd32_getpagesize, where we'll always return 4096 since that's the only
PAGE_SIZE we support, similar to how the ia32 implementation does things.

Reviewed by: manu@
Differential Revision: https://reviews.freebsd.org/D21192