Add ABI compatability aliases.

GNU used several extensions that were incompatible with C99 and POSIX,
so they used alternate names for the standard functions.

The result is that we need these to run standards-conformant programs
that were linked with glibc.

src/string/strerror_r.c

@@ -1,5 +1,6 @@
 #include <string.h>
 #include <errno.h>
+#include "libc.h"
 
 int strerror_r(int err, char *buf, size_t buflen)
 {
@@ -15,3 +16,5 @@ int strerror_r(int err, char *buf, size_t buflen)
 	memcpy(buf, msg, l+1);
 	return 0;
 }
+
+weak_alias(strerror_r, __xpg_strerror_r);