arm64 lib32: enable building of lib32 on arm64

Enable LIB32 option on aarch64, defaulting to YES; it had defaulted
to "broken". Add required variables for how to compile lib32 on
arm. Use /usr/include/arm for armv7 (32-bit) headers, analogous to
/usr/include/i386 on amd64. Omit libomp from lib32; it is not
supported on armv7.

Reviewed by: jrtc27
Differential Revision: https://reviews.freebsd.org/D40945