add getauxval function

in a sense this implementation is incomplete since it doesn't provide
the HWCAP_* macros for use with AT_HWCAP, which is perhaps the most
important intended usage case for getauxval. they will be added at a
later time.

src/misc/getauxval.c

@@ -0,0 +1,12 @@
+#include <sys/auxv.h>
+#include <errno.h>
+#include "libc.h"
+
+unsigned long getauxval(unsigned long item)
+{
+	size_t *auxv = libc.auxv;
+	for (; *auxv; auxv+=2)
+		if (*auxv==item) return auxv[1];
+	errno = ENOENT;
+	return 0;
+}