liburcu: disable build with gcc-4.8.x on ARM

Because of a gcc bug [1], liburcu checks for compiler version [2] and
stops the build prematurely if gcc-4.8.[0-2] is detected for ARM.

However a bug fix exists [3] and may be backported by ARM toolchain
providers; that's what is done in Buildroot [4] or Linaro [5].

So, this change:
- add a patch in liburcu to allow build with ARM gcc-4.8.2;
- adds a new hidden symbol to reflect the toolchain status with regard
  to this bug [1];
- and controls liburcu visibility.

However, a limitation of this is when the user is using a custom
toolchain; that's why the bug details have been added in the liburcu
help message.

[1] http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58854
[2] http://git.lttng.org/?p=userspace-rcu.git;a=commitdiff;h=4b79310
[3] http://gcc.gnu.org/viewcvs/gcc?view=revision&revision=204665
[4] http://git.buildroot.net/buildroot/commit/?id=c443c2be1768ebbdcb76c55d0a08fd7c983488c8
[5] http://bazaar.launchpad.net/~linaro-toolchain-dev/gcc-linaro/4.8/revision/122147



[Peter: hide normal toolchain comment if bug, add comment explaining issue if bug]
Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>