rtld: Add stubs to support PT_GNU_STACK

rtld: Add stubs to support PT_GNU_STACK

This functionality requires kernel support to determine the initial stack
access mode.  This is how it's supposed to work:

If the loaded DSO requires an executable stack as specified by the PF_X
bit or p_flags on PT_GNU_STACK phdr, yet the current stack protection does
not permit execution: The map_stacks_exec function should change the
stack protection mode of all thread stacks.

The PT_GNU_STACK phdr parser has been implemented as well as a private
interface to _rtld_get_stack_prot() in order to export the stack access
mode calculated by rtld.

Left to do:
1) Implement stack protection in the kernel
2) Add PT_GNU_STACK functionality in the kernel
3) Add a method to change stack protection mode in libc or libthread
4) Implement map_stacks_exec function in rtld.