Make the "-m32" loader compile without 32-bit libc headers.

This removes an annoying build dependency on 64-bit systems.
Fix "out of source tree" build support.
Fix corner cases in shebang support.

Broken corner cases were:

- more than 128 characters are specified (this limit is specific to
  the Linux kernel);

- no program is specified; and

- recursive shebang
Make the loader compile with GCC version < 4.5.0 (__builtin_unreachable).
Enable postponed tests and disable deprecated tests.
Remove confusing warnings about placeholder cleanup.