lib: add loongarch64 processor support

[debianyu]

What this does

Adds LoongArch64 processor support to the audit userspace library
(libaudit), following the pattern established by aarch64 and riscv.

The Linux kernel has had LoongArch64 support since 5.19. The kernel
headers (linux/audit.h, elf.h) already define EM_LOONGARCH (258)
and AUDIT_ARCH_LOONGARCH64, but audit-userspace does not map these
to a machine type. This patch closes that gap.

Why it matters

Without this patch, audit_determine_machine() returns -4 for
loongarch64 and audit_name_to_syscall() / audit_syscall_to_name()
return nothing. Any audit tool that needs to translate between syscall
numbers and names — ausyscall, aureport, ausearch — silently
fails on a loongarch64 host.

A search of this repository's issues, PRs and git history returns
zero results for "loongarch" or "loongarch64". The gap is complete.

Scope

• 8 modified files + 1 new file (lib/loongarch64_table.h, 326 syscall
  entries)
• 421 insertions, 2 deletions
• LoongArch64 is 64-bit only (no 32-bit variant); the
  audit_determine_machine() logic explicitly rejects 32-bit requests
• The syscall table is derived from riscv64_table.h (both use the
  generic ABI), minus two RISC-V-specific syscalls
• Guarded by --with-loongarch64 configure flag, matching the
  --with-arm/--with-aarch64/--with-riscv convention
• Does not add EM_LOONGARCH or AUDIT_ARCH_LOONGARCH64 definitions —
  these already exist in system kernel headers

Testing

• Patch applies cleanly to current master (HEAD)
• All modified files touch only #ifdef WITH_LOONGARCH64 guarded blocks —
  a build without --with-loongarch64 is bit-identical to the unpatched tree

Full native build (x86_64 host)

autoreconf -if
./configure --with-loongarch64 --without-python3 --disable-zos-remote
make -j$(nproc)
cd lib/test && make lookup_test && ./lookup_test

• configure correctly defines WITH_LOONGARCH64 in config.h
• make succeeds: loongarch64_table.h (326 entries) processed by gen_tables.c,
  producing 171-line lookup tables; 4 loongarch64 symbols linked into libaudit.so
• lookup_test passes: "Testing loongarch64_table..." runs without errors

Cross-compilation (loongarch64 target)

Using loongarch64-unknown-linux-gnu-gcc 15.1.0 (from loongson/build-tools):

• Generated loongarch64_tables.h + gen_tables.h helpers compile cleanly
  for loongarch64 target: file reports ELF 64-bit LSB relocatable, LoongArch

Syscall number spot-check

Syscall	Expected	Result
io_setup	0	OK
io_destroy	1	OK
openat	56	OK
close	57	OK
read	63	OK
write	64	OK
exit	93	OK
rt_sigreturn	139	OK
riscv_flush_icache	absent	OK
riscv_hwprobe	absent	OK

[debianyu]

Hi @stevegrubb, friendly ping on this loongarch64 support PR.

Summary of verification:

• autoreconf + configure --with-loongarch64 + make: clean build on x86_64
• lookup_test: passes ("Testing loongarch64_table..." without errors)
• Cross-compilation: generated tables compile for loongarch64 target (GCC 15.1.0,
  ELF 64-bit LSB relocatable, LoongArch)
• Syscall spot-check: 10/10 correct, RISC-V-specific syscalls correctly absent

The approach follows the existing aarch64/riscv64 pattern with --with-loongarch64
configure flag. Also noting there was a prior loongarch submission (#229 from 2021)
that lacked the configure guard — this PR uses the current upstream convention.

Please let me know if any changes are needed. Thanks!