Fix eglib on AIX/PASE (mono/mono#17476)

Fix eglib on AIX/PASE (mono/mono#17476)

* fix eglib on AIX/PASE

	* Work around access(2) syscall bug on PASE
	  On PASE, X_OK will always returns true. This is contrary to
	  how AIX behaves (though corresponds to documentation!). This
	  is probably because it's an upcall to the ILE version of
	  access, and that seemingly always returns true.

	* Always export the dummy function in the test suite
	  The AIX linker didn't export this properly, without using the
	  giant sledgehammer of -bexpall. Write an export file and use
	  it. This required some build system changes to expose an
	  AIX-only automake conditional.

	* Use a file that exists on PASE for tests
	  PASE doesn't use Unix-style DNS resolution, so /etc/hosts is
	  missing. Use /etc/magic, since that seems to be present.

	* Don't use /bin on AIX/PASE for tests
	  This is a symlink to /usr/bin, and it seems chdir/getcwd
	  follows links. Use /usr instead, that is unlikely to be a
	  symlink.

	* Make a note what directory we're in if we fail test_cwd

* Change manual executable access check change per Jay

* Change test on POSIXy platforms to always use root directory

Commit migrated from https://github.com/mono/mono/commit/638e2c6a8614d25aad591112d3414e28641b97e0