[lldb] [test/Register] Add read/write tests for x87 regs

Add a partial read/write tests for x87 FPU registers.  This includes
reading and writing ST registers, control registers and floating-point
exception data registers (fop, fip, fdp).

The tests assume the current (roughly incorrect) behavior of reporting
the 'abridged' 8-bit ftag state as 16-bit ftag.  They also assume Linux
plugin behavior of reporting fip/fdp split into halves as (fiseg, fioff)
and (foseg, fooff).

Differential Revision: https://reviews.llvm.org/D88583