PE executables don't seem to have anything in the symbol table. Don't
assert that we find any symbols in this case, which allows using
symcheck for executable binaries.

Implement the following logic:

* For elf executable binaries, check for PT_GNU_STACK with PF_X. This
  combination tells the kernel to make the stack executable.
* For elf intermediate objects, check for `.note.GNU-stack` with
  SHF_EXECINSTR. This combination in an object file tells the linker to
  give the final binary PT_GNU_STACK PF_X.
* For elf intermediate objects with no `.note.GNU-stack`, assume the
  legacy behavior that assumes an executable stack is required.
* For non-elf binaries, don't check anything. In a follow up it may be
  possible to check for `MH_ALLOW_STACK_EXECUTION` on Mach-O binaries,
  but it doesn't seem possible to get the latest compiler to emit this.

This appears to match what is done by `scanelf` to emit `!WX` [1], which
seems to be the tool used to create the output in the issue. The ld
manpage [2] also has some useful notes about these flags, as does the
presentation at [3].

Closes: rust-lang#183

[1]: https://github.com/gentoo/pax-utils/blob/9ef54b472e42ba2c5479fbd86b8be2275724b064/scanelf.c
[2]: https://man7.org/linux/man-pages/man1/ld.1.html
[3]: https://www.ndss-symposium.org/wp-content/uploads/6D-s0924-ye.pdf