[lldb] [Process/elf-core] Fix reading FPRs from FreeBSD/i386 cores

The FreeBSD coredumps from i386 systems contain only FSAVE-style
NT_FPREGSET.  Since we do not really support reading that kind of data
anymore, just use NT_X86_XSTATE to get FXSAVE-style data when available.

Differential Revision: https://reviews.llvm.org/D101086