api: add seccomp_arch_name() to resolve an arch token to its name

[rawrmonster17]

Problem

There is no public API to resolve a SCMP_ARCH_* token back to its
architecture name string. seccomp_arch_resolve_name() converts a name to a
token, but the inverse direction requires callers to maintain their own
token-to-string table, duplicating the mapping already present inside
libseccomp.

Fixes: Github Issue #295

Fix

Add seccomp_arch_name(uint32_t arch_token) as the inverse of
seccomp_arch_resolve_name():

• Returns a pointer to a static string constant (caller must not free).
• SCMP_ARCH_NATIVE resolves to the native architecture name, matching
  the zero-token convention used by seccomp_arch_add() and
  seccomp_arch_exist().
• Returns NULL for an unrecognized token.

Implementation

A new internal helper arch_def_name() in src/arch.c holds the single
authoritative token→name table and is declared in src/arch.h.

The existing static _pfc_arch() in src/gen_pfc.c previously contained
an identical switch — it is now a two-line wrapper calling arch_def_name(),
eliminating the duplicate table and the linux/audit.h include that was
required solely for that switch.

The public seccomp_arch_name() in src/api.c calls arch_def_name()
directly, following the same API visibility macro pattern as every other
public arch function.

Documentation

seccomp_arch_add(3) is updated (NAME, SYNOPSIS, DESCRIPTION, RETURN VALUE)
and a .so redirect seccomp_arch_name(3) is added, following the same
pattern as seccomp_arch_resolve_name(3) and every other arch function stub.

Tests

New test 65-basic-arch_name round-trips every known SCMP_ARCH_* token
through seccomp_arch_resolve_name() and back through seccomp_arch_name(),
verifies SCMP_ARCH_NATIVE resolves to the native arch name, and confirms
that an unrecognized token (0xdeadbeef) returns NULL.

Testing

• Zero new compiler warnings (-Wall -Wextra).
• make check passes (PASS: arch-syscall-check, PASS: regression).
• ./tests/65-basic-arch_name passes directly.