[lldb] Add explicit 64-bit fip/fdp registers on x86_64

The FXSAVE/XSAVE data can have two different layouts on x86_64.  When
called as FXSAVE/XSAVE..., the Instruction Pointer and Address Pointer
registers are reported using a 16-bit segment identifier and a 32-bit
offset.  When called as FXSAVE64/XSAVE64..., they are reported using
a complete 64-bit offsets instead.

LLDB has historically followed GDB and unconditionally used to assume
the 32-bit layout, with the slight modification of possibly
using a 32-bit segment register (i.e. extending the register into
the reserved 16 upper bits).  When the underlying operating system used
FXSAVE64/XSAVE64..., the pointer was split into two halves,
with the upper half repored as the segment registers.  While
reconstructing the full address was possible on the user end (and e.g.
the FPU register tests did that), it certainly was not the most
convenient option.

Introduce a two additional 'fip' and 'fdp' registers that overlap
with 'fiseg'/'fioff' and 'foseg'/'foff' respectively, and report
the complete 64-bit address.

Differential Revision: https://reviews.llvm.org/D91497

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