fbsd-nat: Use regset supply/collect methods.

fbsd-nat: Use regset supply/collect methods.

fbsd-nat includes various helper routines for fetching and storing
register sets via ptrace where the register set is described by a
regset.  These helper routines directly invoke the
supply/collect_regset regcache methods which doesn't permit a regset
to provide custom logic when fetching or storing a register set.
Instead, just use the function pointers from the struct regset
directly.

Approved-By: Simon Marchi <simon.marchi@efficios.com>