Round XML register bitsize to byte boundary

This patch allows LLDB to accept register sizes which are not aligned
to 8 bits bitsize boundary. This fixes a crash in LLDB when connecting
to OpenOCD stub. GDB xml description allows for non-aligned bit lengths
but they are rounded off to nearest byte during transfer. In case of
OpenOCD some of SOC specific system registers were less than a single
byte in length and were causing LLDB to crash.

Reviewed By: labath

Differential Revision: https://reviews.llvm.org/D111131