mips64: clean up register context storage

Store the gpr data in a DataBufferHeap and use a DataExtractor to
extract register values with appropriate endianness.  This avoids hard-
coding the register count, and with some further work would allow this
class to provide generic register context storage for any CPU.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@205329 91177308-0d34-0410-b5e6-96231b3b80d8