Power PC Fix extraction of the L field for sync instruction

Power PC Fix extraction of the L field for sync instruction

The L field is currently a two bit[22:21] field in ISA 3.0. The size of the
L field has changed over time.

Currently the ISA 3.0 Valgrind sync instruction support code sets the
flag_L for the instruction L field to a five bit value that includes bits
that are marked reserved the sync instruction.  This patch fixes the issue for ISA 3.0
to only setting flag_L the specified two bits.

Valgrind bugzilla:   https://bugs.kde.org/show_bug.cgi?id=422677

(cherry picked from commit fb6f7abcbc92506d302fb18a2c5fc853d2929248)