[LLDB][MIPS] Use the correct ptrace buffer for writing register value for o32 applications

For o32 applications on mips we were getting segmentation fault while launching lldb-server because of overwritting stack when using elf_gregset_t in DoWriteRegisterValue.
We are now using the GPR_mips_linux buffer in DoWriteRegisterValue as done in DoReadRegisterValue also, which solves the above issue.

llvm-svn: 250696