[gdb/tdep] Use SYSCALL_MAP_RENAME for aarch64 and loongarch

[gdb/tdep] Use SYSCALL_MAP_RENAME for aarch64 and loongarch

There are currently two functions using macros SYSCALL_MAP and
UNSUPPORTED_SYSCALL_MAP: aarch64_canonicalize_syscall, and
loongarch_canonicalize_syscall.

Here [1] I propose to do the same in i386_canonicalize_syscall, using one
additional macro: SYSCALL_MAP_RENAME.

Add the same macro in aarch64_canonicalize_syscall and
loongarch_canonicalize_syscall, and use it to map aarch64_sys_mmap and
loongarch_sys_mmap to gdb_sys_mmap2.

While we're at it:
- reformat the macro definitions to be more readable,
- add missing macro undefs in aarch64_canonicalize_syscall, and
- fix indentation in aarch64_canonicalize_syscall.

No functional changes.

Tested by rebuilding on x86_64-linux.

Reviewed-By: Alexandra Petlanova Hajkova <ahajkova@redhat.com>
[1] https://sourceware.org/pipermail/gdb-patches/2025-March/216230.html