build(atomic): detect GCC/Clang __atomic builtins, enabling native atomics

[gburd]

What

Teach configure to detect the GCC/Clang __atomic builtins and select them
as the atomic-operations backend, ahead of the inline-assembly tier.

Why

The atomic-operation detection in dist/aclocal/mutex.m4 only knew the
x86/gcc-assembly, Solaris, and MinGW tiers. On a modern clang or gcc toolchain
the x86/gcc-assembly probe runs first and fails to compile (its probe body
calls exit() with no <stdlib.h>, a hard error under C99+). Detection then
falls through every remaining tier and db_cv_atomic ends up no.

With HAVE_ATOMIC_SUPPORT undefined, the engine uses the mutex-emulated atomic
tier: atomic_inc/atomic_dec/atomic_compare_exchange on mpool buffer
refcounts, shared-latch reader counts, and lock-manager counters each take a
pool mutex. Native hardware atomics were silently off on every modern build.

This is the same class of issue as the K&R / C-standard fix in #25.

Change

• Add a __atomic builtin probe (tests __atomic_add_fetch plus 32-bit and
  pointer-width __atomic_compare_exchange_n) and try it before the inline-asm
  probe. The builtin tier is portable across architectures and provides the
  full set of primitives the engine uses, including 64-bit and pointer-width
  compare-and-swap.
• When it matches, define HAVE_ATOMIC_SUPPORT, HAVE_ATOMIC_BUILTINS, and
  HAVE_ATOMIC_GCC_BUILTIN, and probe 64-bit support so HAVE_ATOMIC_64BIT is
  set.
• HAVE_ATOMIC_GCC_BUILTIN is the macro os_ext.h uses to expose the
  __os_atomic_* prototypes; defining it from configure makes those
  prototypes visible to every translation unit, not just os_atomic.c.
• Replace the bare exit(0) in the x86/gcc-assembly probe with (void)0 so it
  compiles cleanly on modern toolchains.
• configure and config.hin regenerated with Autoconf 2.73.

No engine source changes; this only selects which existing os_atomic.c tier
compiles.

Verification

On x86_64-linux / clang, building from clean master plus these files:

• checking for atomic operations... gcc/__atomic
• checking for 64-bit atomic operations... yes
• db_config.h: HAVE_ATOMIC_SUPPORT, HAVE_ATOMIC_BUILTINS,
  HAVE_ATOMIC_GCC_BUILTIN, HAVE_ATOMIC_64BIT all defined
• full build clean; __os_atomic_cas_ptr compiles to a single lock cmpxchg
  with no mutex call
• TCL: test001, test012, ssi001, ssi002 pass; recd001 recovery cycles verify