Unify x86/x86-64 ABI handling in SIMD prologues and CPU-feature probes

[Xor-el]

Centralizes the x86-64 ABI decision behind a single HASHLIB_SYSV_X64_ABI
define, deduplicates non-volatile register save/restore across SIMD
assembly bodies, trims branches not reachable in practice, and tightens
the CPU-feature probe shim. No algorithmic changes.

Key changes

• HashLib.inc defines HASHLIB_SYSV_X64_ABI when HASHLIB_X86_64 AND NOT HASHLIB_MSWINDOWS.
  All SIMD prologues and CPU-feature shims now gate SysV remapping on
  this symbol, decoupling ABI from compiler vendor and fixing the
  previously-broken assumption that "non-Windows ⇒ FPC."
• SimdProc{1..6}Begin_x86_64.inc collapse their nested {$IFDEF FPC}
  / {$IFDEF MSWINDOWS} structure to two flat axes (frame directive,
  ABI). MS-ABI stack-load lines in files 5 and 6 are now written once.
• SimdProc{1..6}Begin_i386.inc collapse the identical FPC and
  Delphi-Win32 branches (files 1–3) and drop dead Delphi-non-Windows
  branches (files 4–6).
• All x86-64 SIMD bodies (Adler32, Argon2, BLAKE2/3, SHA-1/256/512,
  Keccak, SHA-NI) now save/restore XMM6+, RDI, RSI unconditionally
  rather than under {$IFDEF MSWINDOWS}. Comments updated accordingly.
• i386 SIMD bodies save XMM6/XMM7 unconditionally, with comments noting
  this is defensive (i386 has no caller-saved XMM contract).
• CpuIdQuery.inc / XGetBvQuery.inc switch to HASHLIB_SYSV_X64_ABI,
  rewrite the IA-32 entry-state comments to show registers at entry
  rather than mid-shuffle, and gain inline ABI-normalization comments.
• TX86SimdFeatures: TCpuIdResult moved to implementation-section
  type; CpuIdQuery / XGetBvQuery forward declarations take typed
  out parameters instead of Pointer; six identical kill-blocks in
  ApplyBuildOverrides factored into DisableAllExtraFeatures.
• Blake3_HashMany_Scalar comment reworded — the routine is correct on
  any little-endian target, not specifically x86.

What didn't change

• No instruction sequences in any SIMD body modified.
• Byte-identical digests on every test vector across every SIMD tier.
• No public API changes to TX86SimdFeatures.
• No new conditional defines required from callers.