Implement ptrace get registers for aarch64.

Modify the mips code to use the current getregs code path.

Create a new configuration value for getregset to be used by aarch64.

Change-Id: I014c7418937619485ef6c1a0a8b3db8acc4fc89c
Fix mips memory reads.

Cast the first 32 bit read to an unsigned value to avoid sign extending the
value up to 64 bits.

Change-Id: Ib678f48acf91f382fd5d8bcbf2be7d5803aaf0b8