rtasm: add minimal x86-64 support and new instructions (v5)

rtasm: add minimal x86-64 support and new instructions (v5)

Changes in v5:
- Add sse2_movdqa

Changes in v4:
- Use _WIN64 instead of WIN64

Changes in v3:
- Add target and target caps functions, so that they could be different in
  principle from the current CPU and they don't need #ifs to check

Changes in v2:
- Win64 support (untested)
- Use u_cpu_detect.h constants instead of #ifs

This commit adds minimal x86-64 support: only movs between registers
are supported for r8-r15, and x64_rexw() must be used to ask for 64-bit
operations.

It also adds several new instructions for the new translate_sse code.

movdqa