libc

- fix random failures for the "stdlib" and "unistd" tests
- pass the FIXME test
Platform specific:
- FreeBSD:
* fix build (conflict with /usr/local/include)
* complete _start(), signal() on amd64
* check syscall() on amd64
* reflect the current status in the test suite
- Linux:
* fork() and pause() may have to be inline (asm/unistd.h)
* re-introduce ARM OABI
- OpenBSD:
* complete support for SSP