android: Fix ELF TLS support.

Android 29 introduced general-dynamic TLS variable support ("quick
function call to look up the location of the dynamically allocated
storage"), while Mesa on normal Linux has used initial-exec ("use some of
the startup-time fixed slots, hope for the best!").  Both would be better
options than falling all the way back to pthread_getspecific(), which is
the alternative we have pre-29.

Reviewed-by: Roman Stratiienko <r.stratiienko@gmail.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10389>

src/util/u_thread.h

@@ -77,6 +77,15 @@
  */
 #ifdef _MSC_VER
 #define __THREAD_INITIAL_EXEC __declspec(thread)
+#elif defined(ANDROID)
+/* Android 29 gained ELF TLS support, but it doesn't support initial-exec and
+ * it will throw:
+ *
+ *     dlopen failed: TLS symbol "(null)" in dlopened
+ *     "/vendor/lib64/egl/libEGL_mesa.so" referenced from
+ *     "/vendor/lib64/egl/libEGL_mesa.so" using IE access model.
+ */
+#define __THREAD_INITIAL_EXEC __thread
 #else
 #define __THREAD_INITIAL_EXEC __thread __attribute__((tls_model("initial-exec")))
 #endif