linux-libc-headers: update to v3.8

linux-libc-headers: update to v3.8

Now that the 3.8 kernel has been released we can bump the libc-headers
to that version and remove the 3.7 variant. Userspace compatibility is
maintained through kernel versions, we also make the single 3.8 version
the toolchain default.

(From OE-Core rev: 4f9ef639143d890e9d2e71fea3b461fcc8e3f678)

Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>