arch/riscv/riscv_features.c: fix uclibc build

Fix the following uclibc build failure raised since version 2.1.4 and
zlib-ng@6ff8b52:

/home/autobuild/autobuild/instance-2/output-1/build/zlib-ng-2.1.6/arch/riscv/riscv_features.c:4:10: fatal error: sys/auxv.h: No such file or directory
    4 | #include <sys/auxv.h>
      |          ^~~~~~~~~~~~

Fixes:
 - http://autobuild.buildroot.org/results/06a7d8e59ec4de7c711d3f4a4624f67b97d78afe

Signed-off-by: Fabrice Fontaine <[email protected]>

arch/riscv/riscv_features.c

@@ -1,9 +1,12 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
-#include <sys/auxv.h>
 #include <sys/utsname.h>
 
+#if defined(__linux__) && defined(HAVE_SYS_AUXV_H)
+#  include <sys/auxv.h>
+#endif
+
 #include "zbuild.h"
 #include "riscv_features.h"
 
@@ -33,7 +36,11 @@ void Z_INTERNAL riscv_check_features_compile_time(struct riscv_cpu_features *fea
 }
 
 void Z_INTERNAL riscv_check_features_runtime(struct riscv_cpu_features *features) {
+#if defined(__linux__) && defined(HAVE_SYS_AUXV_H)
     unsigned long hw_cap = getauxval(AT_HWCAP);
+#else
+    unsigned long hw_cap = 0;
+#endif
     features->has_rvv = hw_cap & ISA_V_HWCAP;
 }