Add remote bitbang write buffer.

Add remote bitbang write buffer.

Change 7dd323b26 reduced remote bitbang performance a lot. This change
gets most of that performance back again, by reintroducing a write
buffer.

Performance numbers collected using DebugBreakpoint test from
riscv-tests/debug against a single 64-bit spike (RISC-V simulator)
instance. (Ubuntu 20.04.2, AMD Ryzen 5 3600)
Before Windows support was added: 3.09s
After Windows support was added: 12.67s
After this change: 4.69s

Signed-off-by: Tim Newsome <tim@sifive.com>
Change-Id: I72ff4912cbbf316a30ef065e5b8f461a555f06cc
Reviewed-on: http://openocd.zylin.com/6283
Tested-by: jenkins
Reviewed-by: Jan Matyas <matyas@codasip.com>
Reviewed-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>