[lldb] [Process/FreeBSDRemote] Access FPR via RegisterInfo offsets

Use offset-based method to access base x87 FPU registers, using offsets
relative to the position of 'struct FPR', as determined by the location
of first register in it (fctrl).  Change m_fpr to use a fixed-size array
matching FXSAVE size (512 bytes).  Add unit tests for verifying
RegisterInfo offsets and sizes against the FXSAVE layout.

Differential Revision: https://reviews.llvm.org/D91248