use "glibc errno" trick not only for ash, but for entire busybox

(add/remove: 1/1 grow/shrink: 37/37 up/down: 139/-228)        Total: -89 bytes

include/libbb.h

@@ -165,6 +165,14 @@
 #endif
 
 
+#if defined(__GLIBC__)
+/* glibc uses __errno_location() to get a ptr to errno */
+/* We can just memorize it once - no multithreading in busybox :) */
+extern int *const bb_errno;
+#undef errno
+#define errno (*bb_errno)
+#endif
+
 #if defined(__GLIBC__) && __GLIBC__ < 2
 int vdprintf(int d, const char *format, va_list ap);
 #endif