update musl version to 1.1.20

Makefile

@@ -4,7 +4,7 @@ SOURCES = sources
 CONFIG_SUB_REV = 3d5db9ebe860
 BINUTILS_VER = 2.27
 GCC_VER = 6.4.0
-MUSL_VER = 1.1.19
+MUSL_VER = 1.1.20
 GMP_VER = 6.1.1
 MPC_VER = 1.0.3
 MPFR_VER = 3.1.4