Extract DNS hash to separate header with ARM64 support

[vstakhov]

Summary

• Extract platform-specific hash functions from rbldnsd_dnhash.c to reusable dns_hash.h header
• Add ARM64 (aarch64) CRC32-C support via ACLE intrinsics
• Simplify implementation by removing unnecessary complexity for long strings (DNS max is 255 bytes)
• Replace t1ha dependency with inline FNV-1a fallback

Changes

Platform	Implementation	Intrinsics
x86_64	CRC32-C via SSE4.2	_mm_crc32_u64/u8
aarch64	CRC32-C via ACLE	__crc32cd/cb
Other	FNV-1a	(portable)

Build Requirements

• x86_64 packages: Require SSE4.2 (-msse4.2)
• aarch64 packages: Require ARMv8.1+ or ARMv8-A+crc (-march=armv8-a+crc)
• Source builds: Will use FNV-1a fallback if hardware CRC unavailable

Line count

Before: 832 lines (rbldnsd_dnhash.c with embedded hash)
After:  600 lines (rbldnsd_dnhash.c) + 153 lines (dns_hash.h)
Net:    ~80 lines removed, cleaner separation

Test plan

• Build on x86_64 Linux with -msse4.2
• Build on ARM64 Linux with -march=armv8-a+crc
• Build without arch flags (fallback path)
• Run existing functional tests