tcmode: drop en_US.UTF-8 from GLIBC_GENERATE_LOCALES

This locale doesn't seem to be available in the external toolchain, so make 
sure it isn't included in this variable, otherwise locale-base will depend 
upon a non-existant package.

JIRA: SB-2602

Signed-off-by: Christopher Larson <kergoth@gmail.com>