[lldb] [lit] Introduce tests for reading x86 general purpose registers

Introduce tests for reading the eight x86 general purpose registers,
i.e. RAX/RBX/RCX/RDX/RBP/RSP/RSI/RDI and their shorter counterparts.
The test comes in separate 32-bit and 64-bit variant, targeting
appropriate processors.

While technically the 32-bit test could run on amd64, it would be
redundant to the 64-bit version, so just run one of them on each arch.

Differential Revision: https://reviews.llvm.org/D61210

llvm-svn: 359438