target-alpha: get rid of tests on env->user_mode_only

Replace runtime checks on env->user_mode_only by compile time
checks on CONFIG_USER_ONLY.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6277 c046a42c-6fe2-441c-8c8c-71466251a162
