Support SHA-512 hwcap detection on old libc

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>

library/sha512.c

@@ -102,6 +102,14 @@
 #      if defined(__linux__)
 /* Our preferred method of detection is getauxval() */
 #        include <sys/auxv.h>
+#        if !defined(HWCAP_SHA512)
+/* The same header that declares getauxval() should provide the HWCAP_xxx
+ * constants to analyze its return value. However, the libc may be too
+ * old to have the constant that we need. So if it's missing, assume that
+ * the value is the same one used by the Linux kernel ABI.
+ */
+#          define HWCAP_SHA512 (1 << 21)
+#        endif
 #      endif
 /* Use SIGILL on Unix, and fall back to it on Linux */
 #      include <signal.h>