add execmem build option

The execmem option can be set to false to disable the dynamic
dispatch patching that requires mmap(PROT_WRITE | PROT_EXEC),
which is undesirable on some platforms.

Signed-off-by: John Bates <jbates@chromium.org>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10987>

src/mapi/u_execmem.c

@@ -121,6 +121,10 @@ init_map(void)
 void *
 u_execmem_alloc(unsigned int size)
 {
+#ifndef MESA_EXECMEM
+   (void)size;
+   return NULL;
+#else
    void *addr = NULL;
 
    mtx_lock(&exec_mutex);
@@ -140,6 +144,7 @@ bail:
    mtx_unlock(&exec_mutex);
 
    return addr;
+#endif /* MESA_EXECMEM */
 }