update default musl version to 1.2.1

Author: richfelker

Makefile

@@ -4,7 +4,7 @@ SOURCES = sources
 CONFIG_SUB_REV = 3d5db9ebe860
 BINUTILS_VER = 2.33.1
 GCC_VER = 9.2.0
-MUSL_VER = 1.2.0
+MUSL_VER = 1.2.1
 GMP_VER = 6.1.2
 MPC_VER = 1.1.0
 MPFR_VER = 4.0.2