[lldb] [gdb-remote] Fix displaying i387_ext & vec regs with gdbserver

Adjust the encoding and format applied to i387_ext and vec* type
registers from gdbserver to match lldb-server.  Both types are now
displayed as vector of uint8 instead of float and integer formats used
before.  Additionally, this fixes display of STi registers when they do
not carry floating-point data (they are also used to hold MMX vectors).

Differential Revision: https://reviews.llvm.org/D108468