arm: Unbreak debugging programs that use FP instructions

Contrary to arm64, on armv7 get_vfpcontext/set_vfpcontext can be called
from cpu_ptrace. This can be triggered when gdb hits a breakpoint
in a userspace program.
Relax td == currthread assertion to account for that situation.
While here update an outdated comment in vfp_discard.

Reported by: Mark Millard <marklmi@yahoo.com>
Tested by: Mark Millard <marklmi@yahoo.com>
Differential Revision: https://reviews.freebsd.org/D38696