elf: early conversion of elf p_flags to mprotect flags

This patch replaces _dl_stack_flags global variable by
_dl_stack_prot_flags.
The advantage is that any convertion from p_flags to final used mprotect
flags occurs at loading of p_flags. It avoids repeated spurious
convertions of _dl_stack_flags, for example in allocate_thread_stack.

This modification was suggested in:
  https://sourceware.org/pipermail/libc-alpha/2025-March/165537.html

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

sysdeps/unix/sysv/linux/dl-execstack.c

@@ -17,6 +17,7 @@
    <https://www.gnu.org/licenses/>.  */
 
 #include <ldsodefs.h>
+#include <stackinfo.h>
 
 int
 _dl_make_stack_executable (const void *stack_endp)
@@ -36,7 +37,7 @@ _dl_make_stack_executable (const void *stack_endp)
     return errno;
 
   /* Remember that we changed the permission.  */
-  GL(dl_stack_flags) |= PF_X;
+  GL(dl_stack_prot_flags) |= PROT_EXEC;
 
   return 0;
 }