uclibc-ng doesn't provide getauxval which results in the following build
failure on arm or ppc with any user of abseil-cpp such as grpc:

/home/buildroot/autobuild/instance-0/output-1/host/opt/ext-toolchain/bin/../lib/gcc/arm-buildroot-linux-uclibcgnueabi/10.3.0/../../../../arm-buildroot-linux-uclibcgnueabi/bin/ld: /home/buildroot/autobuild/instance-0/output-1/host/arm-buildroot-linux-uclibcgnueabi/sysroot/usr/lib/libabsl_random_internal_randen_hwaes.so.2111.0.0: undefined reference to `getauxval'

To fix this build failure, check that __UCLIBC__ is not defined before
using getauxval (as Babel is not able to check function availability)

Fixes:
 - http://autobuild.buildroot.org/results/775f3ca3dedebff29e212b29dfa896b7613b7a02

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>