FreeBSD/x86: Read segment base registers from NT_X86_SEGBASES.

FreeBSD/x86: Read segment base registers from NT_X86_SEGBASES.

FreeBSD kernels recently grew a new register core dump note containing
the base addresses of the %fs and %gs segments (corresponding to the
%fsbase and %gsbase registers).  Parse this note to permit inspecting
TLS variables in core dumps.  Native processes already supported TLS
via older ptrace() operations.