perf annotate: Add riscv64 support

This patch adds basic arch initialization and instruction associate
support for the riscv64 CPU architecture.

Example output:

  $ perf annotate --stdio2
  Samples: 122K of event 'task-clock:u', 4000 Hz, Event count (approx.): 30637250000, [percent: local period]
  strcmp() /usr/lib64/libc-2.32.so
  Percent

	      Disassembly of section .text:

	      0000000000069a30 <strcmp>:
	      __GI_strcmp():
	      const unsigned char *s2 = (const unsigned char *) p2;
	      unsigned char c1, c2;

	      do
	      {
	      c1 = (unsigned char) *s1++;
   37.30        lbu  a5,0(a0)
	      c2 = (unsigned char) *s2++;
    1.23        addi a1,a1,1
	      c1 = (unsigned char) *s1++;
   18.68        addi a0,a0,1
	      c2 = (unsigned char) *s2++;
    1.37        lbu  a4,-1(a1)
	      if (c1 == '\0')
   18.71      ↓ beqz a5,18
	       return c1 - c2;
	       }

Signed-off-by: William Cohen <wcohen@redhat.com>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: linux-riscv@lists.infradead.org
Link: http://lore.kernel.org/lkml/20210927005115.610264-1-wcohen@redhat.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>