bsd.cpu.mk: Simplify armv[67] support

Drop a clause that's no longer relevant to v4/v5. Drop support for
softfloat for v[45]. Simplify soft float expression by assuming we're
always either armv6 or armv7.

Sponsored by: Netflix
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D38584