Fix support to execute 32-bit programs from 64-bit programs.

As stated in the is_32on64_mode() function, 32-bit/64-bit mode change
is effective immediately.  This was effective for 64-bit -> 32-bit
change, but not for the other way.

Reported-by: Erwan GOURIOU <erwan.gouriou@st.com> & Sebastien GANDON <sebastien.gandon@st.com>
Investigated-by: Yann Droneaud <yann@droneaud.fr>
Ref.: https://github.com/cedric-vincent/PRoot/issues/73
Add test for commit c47aeb7d "Fix ptrace emulation for multi-threaded programs"