share/mk: Fix a heuristic in bsd.cpu.mk

When cross-building from MacOS we have MACHINE=arm64 MACHINE_ARCH=arm,
so bsd.cpu.mk infers that sizeof(long) == 4, but of course it isn't.
This breaks the bootstrap build of openssl, which tests
MACHINE_ABI:Mlong64 to decide whether to build ecp_nistp224.c. It
doesn't, and crypto/openssl/freebsd/include/openssl/configuration.h
undefines OPENSSL_NO_EC_NISTP_64_GCC_128, so we end up with a link
error.

Reviewed by: ngie, emaste
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D52340