Add an inline fast path to JEMALLOC_TLS_ADDR for static TLS

The noinline accessor puts an opaque call on the malloc/free fastpath.
test/stress/microbench (malloc(1)/free pairs, pinned core, clang):

                     unpatched      accessor       this commit
  malloc, no LTO     7.0 ns/op      9.8 (+40%)     7.0 (+-0%)
  free,   no LTO     6.5 ns/op      9.2 (+40%)     6.5 (+-0%)
  malloc, ThinLTO    7.0 ns/op      9.0 (+28%)     7.6 (~+8%)
  free,   ThinLTO    6.5 ns/op      8.4 (+28%)     7.0 (~+8%)

Under a static TLS model the tsd address is `thread_pointer + offset` with a
thread-independent offset.  Capture the offset once (a noinline helper, lazy and
sentinel-initialized) and re-read the thread pointer per call with a `volatile`
asm the optimizer may not hoist or CSE (per-arch reads from mimalloc's
mi_prim_tls_slot).  Both inputs are hoist-proof: the tp read is volatile, the
offset is thread-independent.

The offset global is read and written with relaxed atomics: threads racing
their first allocation init it concurrently with the same value, so a plain
access would be a benign-but-UB data race (TSan-reported).  Relaxed adds no
fence -- it compiles to the same load/store on the fast path.

The offset must NOT be computed inline as `&tsd_tls - __builtin_thread_pointer()`:
the terms hoist independently, and clang ThinLTO keeps the stale `&tsd_tls` in a
callee-saved register while re-reading the thread pointer, so `fresh_tp +
(stale_addr - fresh_tp)` cancels back to the stale address.  Capturing behind a
call boundary evaluates both terms at one point on one thread.

Gated on JEMALLOC_TLS_MODEL_INITIAL_EXEC (a new configure define, set whenever
jemalloc applies its default initial-exec model), GNU asm, a known arch, and
!_WIN32 (MinGW's thread pointer lives in the TEB, not fs/gs:0); everything else
keeps the noinline accessor.  Verified with the fiber-migration reproducer under
clang ThinLTO.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

configure.ac

@@ -2841,6 +2841,8 @@ if test "x${je_cv_tls_model}" = "xyes" -a \
   AC_DEFINE([JEMALLOC_TLS_MODEL],
             [__attribute__((tls_model("initial-exec")))],
             [ ])
+  AC_DEFINE([JEMALLOC_TLS_MODEL_INITIAL_EXEC], [ ],
+            [Defined when the TSD thread-locals use the initial-exec (static) TLS model, i.e. live at a fixed offset from the thread pointer.])
 else
   AC_DEFINE([JEMALLOC_TLS_MODEL], [ ], [ ])
 fi