package/liburcu: update supported architectures

package/liburcu: update supported architectures

- aarch64_be is supported since version 0.10.0 and
  https://github.com/urcu/userspace-rcu/commit/d1c678425415ed986d0101d5d9ab228b160b2fc3
- m68k is supported since version 0.10.0 and
  https://github.com/urcu/userspace-rcu/commit/2748bd6c7b5732f79cea82eba175b91a35d6833f
- mips is supported since version 0.7.4 and
  https://github.com/urcu/userspace-rcu/commit/e1259cb17954e4e03954ba7376d3ed61649116cd
- nios2 is supported since version 0.10.0 and
  https://github.com/urcu/userspace-rcu/commit/859050b3088aa3f0cb59d7f51ce24b9a0f18faa5
- powerpc64 is supported
- sparc64 is supported

Build tested:
                      br-m68k-68040-full [15/44]: OK
                 br-mips32r6-el-hf-glibc [17/44]: OK
                      br-mips64-n64-full [18/44]: OK
                 br-mips64r6-el-hf-glibc [19/44]: OK
                      br-mipsel-o32-full [20/44]: OK
                          br-nios2-glibc [21/44]: OK
             br-powerpc64le-power8-glibc [24/44]: OK
               br-powerpc64-power7-glibc [25/44]: OK
                        br-sparc64-glibc [30/44]: OK
                       linaro-aarch64-be [35/44]: OK

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>