FreeBSD x86 nat: Use register maps for GP register sets.

Rather than using the x86-specific register offset tables, use
register maps to describe the layout of the general purpose registers
fetched via PT_GETREGS.  The sole user-visible difference is that
FreeBSD/amd64 will now report additional segment registers ($ds, $es,
$fs, and $gs) for both 32-bit and 64-bit processes.

As part of these changes, the FreeBSD x86 native targets no longer use
amd64-bsd-nat.c or i386-bsd-nat.c.  Remove FreeBSD-specific register
handling (for $fs_base, $gs_base, and XSAVE state) from these files.
Similarly, remove the global x86bsd_xsave_len from x86-bsd-nat.c.  The
FreeBSD x86 native targets use a static xsave_len instead.

While here, rework the probing of PT_GETXMMREGS on FreeBSD/i386.
Probe the ptrace op once in the target read_description method and
cache the result for the future similar to the way the status of XSAVE
support is probed in the read_description method.  In addition, return
the proper xcr0 mask (X87-only) for old kernels or systems without
either XSAVE or XMM support.