Fix build with uclibc-ng (#1145)

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>

absl/debugging/internal/vdso_support.cc

@@ -33,7 +33,7 @@
 #endif
 #include <unistd.h>
 
-#if defined(__GLIBC__) && \
+#if !defined(__UCLIBC__) && defined(__GLIBC__) && \
     (__GLIBC__ > 2 || (__GLIBC__ == 2 && __GLIBC_MINOR__ >= 16))
 #define ABSL_HAVE_GETAUXVAL
 #endif

absl/random/internal/randen_detect.cc

@@ -24,6 +24,11 @@
 
 #include "absl/random/internal/platform.h"
 
+#if !defined(__UCLIBC__) && defined(__GLIBC__) && \
+    (__GLIBC__ > 2 || (__GLIBC__ == 2 && __GLIBC_MINOR__ >= 16))
+#define ABSL_HAVE_GETAUXVAL
+#endif
+
 #if defined(ABSL_ARCH_X86_64)
 #define ABSL_INTERNAL_USE_X86_CPUID
 #elif defined(ABSL_ARCH_PPC) || defined(ABSL_ARCH_ARM) || \
@@ -31,7 +36,7 @@
 #if defined(__ANDROID__)
 #define ABSL_INTERNAL_USE_ANDROID_GETAUXVAL
 #define ABSL_INTERNAL_USE_GETAUXVAL
-#elif defined(__linux__)
+#elif defined(__linux__) && defined(ABSL_HAVE_GETAUXVAL)
 #define ABSL_INTERNAL_USE_LINUX_GETAUXVAL
 #define ABSL_INTERNAL_USE_GETAUXVAL
 #endif