[AArch64] Handle W registers as pseudo-registers instead of aliases of X registers

[AArch64] Handle W registers as pseudo-registers instead of aliases of X registers

The aarch64 port handles W registers as aliases of X registers. This is
incorrect because X registers are 64-bit and W registers are 32-bit.

This patch teaches GDB how to handle W registers as pseudo-registers of
32-bit, the bottom half of the X registers.

Testcase included.