arc: Set decr_pc_after_break to 2 for ARC 600

In commit "arc: Fix invalid decr_pc_after_break value" it was stated that PC
shouldn't be decremented by 2 after hitting software breakpoint, because in
baremetal case BRK_S instruction doesn't update PC. This introduced a
regression, because this statement is true only for ARC 700, ARC EM and ARC
HS, but is invalid for ARC 600, where PC gets incremented by two bytes (ARC
600 doesn't support 4-byte BRK).

This patch fixes this, by setting decr_pc_after_break to 2, when current
core is ARC 600.

Signed-off-by: Anton Kolesov <Anton.Kolesov@synopsys.com>