Remove duplicated _rand48 implementation. Use stdlib version instead

Originally, there are _rand48 (in libc/bionic/_rand48.c) and __rand48
(in libc/stdlib/_rand48.c) implemented in bionic.  Besides the naming,
the functionality is identical.  This patch removes the duplicated
_rand48.  Also, drand48 and erand48 are modified accordingly.

Change-Id: Ie5761a0a97f45df8538222a77edacb7c3e0125d7
diff --git a/libc/bionic/drand48.c b/libc/bionic/drand48.c
index fd48196..93272cf 100644
--- a/libc/bionic/drand48.c
+++ b/libc/bionic/drand48.c
@@ -15,10 +15,10 @@
 
 #include "rand48.h"
 
-extern unsigned short _rand48_seed[3];
+extern unsigned short __rand48_seed[3];
 
 double
 drand48(void)
 {
-	return erand48(_rand48_seed);
+	return erand48(__rand48_seed);
 }