Make COMPAT_FREEBSD32 part of GENERIC on arm64.

The cloudabi32.ko kernel modules can only be loaded on AMD64 and ARM64
by kernels built with COMPAT_FREEBSD32. Even though COMPAT_FREEBSD32
does not support the execution of native FreeBSD executables, do add it
to GENERIC, to make cloudabi32.ko usable.

According to size(1), this makes the kernel image approximately 0.7%
larger.

Reviewed by: andrew, imp, emaste
Differential Revision: https://reviews.freebsd.org/D13311