[lldb] [Process/FreeBSDRemote] Access GPR via reginfo offsets

Read and write registers from m_gpr using offsets from RegisterInfo
rather than explicit switch-case.  This eliminates a lot of redundant
code, and avoids mistakes such as type mismatches seen recently (wrt
segment registers).  The same logic will be extended to other register
sets in the future.

Make m_gpr an uint8_t std::array to ease accesses.  Ideally, we could
avoid including <machine/reg.h> entirely in the future and instead
get the correct GPR size from Utility/RegisterContextFreeBSD_* somehow.

While at it, modify register set logic to use an explicit enum with
llvm::Optional<>, making the code cleaner and at the same time enabling
compiler warnings for unhandled sets.

Since now we're fully relying on 'struct GPR' defined
in Utility/RegisterContextFreeBSD_* being entirely in sync with
the system structure, add unit tests to verify the field offsets
and sizes.

Differential Revision: https://reviews.llvm.org/D91216