Remove the x86/x64 assembly backend; use compiler intrinsics insteadHEADmaster

x86.asm/x64.asm were already mostly dead weight -- GetAttacks's real
asm body was retired 2026-09-06 (section 7 of
board_representation/MIGRATION.md), and LockDecrement/
CanUseParallelOpcodes/ParallelCompareUlong/ParallelCompareVector had
zero remaining callers (confirmed by grep before deleting anything).
What was still real and load-bearing:

- CountBits/FirstBit/LastBit: popcount/bsf/bsr CDECL calls. Already
  had faster, compiler-intrinsic replacements sitting right next to
  them in chess.h (FastCountBits/FastFirstBit/FastLastBit, via
  __builtin_popcountll/__builtin_ctzll/__builtin_clzll with -mpopcnt)
  that per-file #ifndef CROUTINES blocks in board.c/eval.c/generate.c/
  probe.c/recogn.c were already redirecting individual call sites to.
  Promoted that redirect to the plain names unconditionally
  (#define CountBits FastCountBits etc. in chess.h) and dropped the
  now-always-true CROUTINES guards in those five files -- CountBits'
  real asm body was a Kernighan bit-clearing loop (~23 cycles/op per
  gmake TEST=1's own benchmark), not even O(1), so this is strictly
  faster as well as removing the toolchain dependency.
- LockCompareExchange/LockIncrement: real atomics backing
  AcquireSpinLock/TryAcquireSpinLock/ReleaseSpinLock (util.c) and the
  move-pool allocator (gamelist.c). Replaced with static inline
  wrappers over __sync_val_compare_and_swap/__sync_add_and_fetch --
  same CAS/atomic-add semantics as the retired `lock cmpxchg`/
  `lock xadd` instructions.

Removes the yasm/nasm build dependency entirely: GNUmakefile's
NASM/NASMFLAGS variables, the CROUTINES flag, the .asm->.o build rule,
and x86.o/x64.o from every OBJS list. x86.asm/x64.asm deleted outright
(git history keeps them if ever needed again). The ASM=1 debug mode
(-S -fverbose-asm, a C-compiler assembly-listing dump, unrelated to
the deleted files despite the similar name) is untouched.

Verified: clean build with no yasm/nasm invocation anywhere in the
log, a clean DEBUG=1 TEST=1 self-test run, and node-count parity
against the previous commit on all three curated suites at sd10
(4,003,856 / 22,308,015 / 26,868,016 on both sides) -- confirms this
is a pure substitution with zero behavioral change, not just "doesn't
crash."

Co-Authored-By: Claude Sonnet 5 <[email protected]>

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