boost: fix build on ARC

This commit adds a patch to Boost to make it use the eventfd()
function provided by the C library when uClibc is used, rather than
falling back to using directly the __NR_eventfd system call. This
fixes the build on ARC, which doesn't define __NR_eventfd.

The original problem is that uClibc pretends to be glibc 2.2, which
didn't had eventfd(), so Boost makes the system call
manually. uClibc-ng, in its next release, will pretend to be glibc
2.10 (see
http://cgit.uclibc-ng.org/cgi/cgit/uclibc-ng.git/commit/?id=4ff3a6c8eb91db71d6dc3d2932b66e848bd20ac3),
which will also fix the problem, but requires bumping the uClibc
version, rebuilding the external toolchains, and so on.

Ideally, Boost should be doing a compile test to detect if eventfd()
is available or not, but the Boost build system is so brain-damaged
that doing so would require way too much effort.

Fixes:

  http://autobuild.buildroot.org/results/22b/22b710346d2cd78b7b51cdccd18d670bb6ac5d24/


  and many similar build failures

[Peter: minor tweaks to description]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>