While current_mode() == USR_MODE works on armv7 from PL0 to check
if we're in user mode, current_mode() would require reading a
privileged register on armv8. To work around this, on arm64 we
introduced a 'user_mode' variable. This variable needs to be per
thread now. Rather than starting to pollute thread_info with a
bunch of bools, create a flags field and a TIF_USER_MODE flag to
replace it. Use it on armv7 too for consistency. Also, now that
we need to create a thread_info initializer, add mpidr utilities
for setting thread_info->cpu.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>