glibc: fixed to inject --enable-kernel configure option through extraconfopt

Note:

Because set_confopt() is reseting confopt, we need to inject the configure
option through extraconfopt.
We use set_confopt() within the multilib parse-config to alter architecture
specific settings like arch_machine and arch_target when building the non-
default ABI.

base/glibc/glibc.conf

@@ -111,7 +111,7 @@ var_insert GCC_WRAPPER_INSERT " " "-O"
 # glibc does not like -ffast-math
 var_insert GCC_WRAPPER_REMOVE " " "-ffast-math"
 
-var_append confopt " " "--enable-kernel=2.6.18"
+var_append extraconfopt " " "--enable-kernel=2.6.18"
 
 # Disable glibc internal debuging but build debuging and profile
 # code so we can use this glibc to debug other programs.