[gdb/record] Fix return value for svc in aarch64_record_branch_except_sys

[gdb/record] Fix return value for svc in aarch64_record_branch_except_sys

The Linaro CI reported the following regression in test-case
gdb.reverse/sigall-reverse.exp on aarch64-linux:
...
(gdb) continue^M
Continuing.^M
The next instruction is syscall exit_group.  It will make the program exit. \
  Do you want to stop the program?([y] or n) yes^M
Process record does not support instruction 0xd4000001 at address $hex.^M
Process record: failed to record execution log.^M
^M
Program stopped.^M
__GI__exit (status=status@entry=0) at ../sysdeps/unix/sysv/linux/_exit.c:30^M
30            INLINE_SYSCALL (exit_group, 1, status);^M
(gdb) FAIL: gdb.reverse/sigall-reverse.exp: continue to signal exit
...
due to commit 0cb5fde010a ("[gdb/testsuite] Fix some incorrect uses of
decimal").

[ Note that 0xd4000001 is "svc #0", the syscall instruction. ]

The test fails because it expects "Process record: inferior program stopped":
...
    gdb_test "continue" "Process record: inferior program stopped.*" \
       "continue to signal exit" \
        "The next instruction is syscall exit_group.* program...y. or n. " \
        "yes"
...
instead of "Process record: failed to record execution log".

The problem is caused by the fact that there are two result value domains:
- one for generic record (res < 0, res == 0, and res > 0) and
- one for aarch64 record (AARCH64_RECORD_SUCCESS = 0, AARCH64_RECORD_UNSUPPORTED = 1
  and AARCH64_RECORD_UNKNOWN = 2, and an unnamed -1).

When returning here from aarch64_record_branch_except_sys:
...
             return tdep->aarch64_syscall_record (aarch64_insn_r->regcache,
                                                  svc_number);
...
we transfer from the generic record domain to the aarch64 record domain, converting:
- a value 1, meaning "Process record: inferior program stopped", into
- a value AARCH64_RECORD_UNSUPPORTED, meaning
  "Process record does not support instruction $hex at address $hex.
   Process record: failed to record execution log.".

[ Then again, the transfer of domains might be in aarch64_linux_syscall_record.
It's kind of hard to say, given that typing is not used to distinguish between
the two domains. ]

Fix this by:
- introducing a conversion function to_aarch64_record_result, and
- using it in aarch64_record_branch_except_sys.

Likewise, add a from_aarch64_record_result and use it in
aarch64_process_record.

Also add some documentation to enum aarch64_record_result, and add the missing
-1 value.

AFAICT, the loongarch port needs the same fix, but I can't test that so I
think it's better to only deal with aarch64 in this patch.

Tested on aarch64-linux.

Approved-By: Luis Machado <luis.machado.foss@gmail.com>
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33913