Add support for executing the FreeBSD 1/i386 a.out binaries on amd64.

In particular:

• implement compat shims for old stat(2) variants and ogetdirentries(2);
• implement delivery of signals with ancient stack frame layout and corresponding sigreturn(2);
• implement old getpagesize(2);
• provide a user-mode trampoline and LDT call gate for lcall $7,$0;
• port a.out image activator and connect it to the build as a module on amd64.

The changes are hidden under COMPAT_43.

MFC after: 1 month