feat(rv64): update am to support riscv64-nanos

See merge request projectn/nexus-am!10

riscv64-nemu,cte: support timer interrupt from clint

See merge request projectn/nexus-am!9

feat(rv64): set up rv64 difftest environment

See merge request projectn/nexus-am!8

am,arch,platform,nemu: use $(MAKE) instead of make

See merge request projectn/nexus-am!7

*-nemu,trm: put mainargs at RAM to fix difftest bug

See merge request projectn/nexus-am!6

Rv64 cte

See merge request projectn/nexus-am!5

Riscv32 noop pal

See merge request projectn/nexus-am!4

* UART can only transmit character data, but not keystrobes.
* To do this, we record the last data UART received, By default we
  generate a keydown event. A keyup event is generated when
    * UART receive a different data from the last one
    * or no data is received in 0.5s
* This is not perfect. Key combination can not be detected since a keyup
  event is generated before another keydown event. But it is enough to
  play PAL with UART.

tests,cachetest: add loader test

See merge request projectn/nexus-am!3

Riscv32 noop

See merge request projectn/nexus-am!2

tests,cputest: add volatile to prevent optimization

Upd ref score for three benchmarks

amtest: add `H` for displaying help msg

Amtest msg

Nemu