[LLDB][MIPS] Fix gp register value for o32 applications on 64-bit target

GP registers for o32 applications were always giving zero value because SetType() on the RegisterValue was causing the accessor functions to pickup the value from m_scalar of RegisterValue which is zero.
In this patch byte size and byte order of register value is set at the time of setting the value of the register.

llvm-svn: 249020