Add support for halfword memory accesses.

I'm working with an FPGA hooked to the STM32 FSMC in 16-bit mode, and I want to be able to poke registers in the FPGA from GDB.

Since both reads and writes may have side effects, it's not safe to decompose 16-bit accesses into two 8-bit accesses or promote it to a 32-bit access (which would get decomposed by the FSMC into two 16-bit accesses) since either will generate an extra access.