nolibc/stdlib: Implement `getauxval(3)` function

Previous commits save the address of the auxiliary vector into a global
variable @_auxv. This commit creates a new function 'getauxval()' as a
helper function to get the auxv value based on the given key.

The behavior of this function is identic with the function documented
in 'man 3 getauxval'. This function is also needed to implement
'getpagesize()' function that we will wire up in the next patches.
Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

tools/include/nolibc/stdlib.h

@@ -12,6 +12,7 @@
 #include "types.h"
 #include "sys.h"
 #include "string.h"
+#include <linux/auxvec.h>
 
 struct nolibc_heap {
 	size_t	len;
@@ -108,6 +109,32 @@ char *getenv(const char *name)
 	return _getenv(name, environ);
 }
 
+static __attribute__((unused))
+unsigned long getauxval(unsigned long type)
+{
+	const unsigned long *auxv = _auxv;
+	unsigned long ret;
+
+	if (!auxv)
+		return 0;
+
+	while (1) {
+		if (!auxv[0] && !auxv[1]) {
+			ret = 0;
+			break;
+		}
+
+		if (auxv[0] == type) {
+			ret = auxv[1];
+			break;
+		}
+
+		auxv += 2;
+	}
+
+	return ret;
+}
+
 static __attribute__((unused))
 void *malloc(size_t len)
 {