glibc 2.20/2.21 does not compile on x86_64, so disable for now

target/config/Config.in.libc.choice

@@ -76,11 +76,11 @@ config ADK_TARGET_LIB_UCLIBC_NG_GIT
 
 config ADK_TARGET_LIB_GLIBC_2_21
 	bool "2.21"
-	depends on ADK_TARGET_LIB_GLIBC
+	depends on ADK_TARGET_LIB_GLIBC && !ADK_TARGET_ARCH_X86_64
 
 config ADK_TARGET_LIB_GLIBC_2_20
 	bool "2.20"
-	depends on ADK_TARGET_LIB_GLIBC
+	depends on ADK_TARGET_LIB_GLIBC && !ADK_TARGET_ARCH_X86_64
 
 config ADK_TARGET_LIB_GLIBC_2_19
 	bool "2.19"