elf: early conversion of elf p_flags to mprotect flags

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>