target/arc: fix off-by-one error in arc_save_context()

target/arc: fix off-by-one error in arc_save_context()

While not affecting the function's main purpose, an error has
crept into arc_save_context() that results in logging wrong register
values when the debug level is 3 or more. For instance, when debugging a
trivial program and halting at entry to main, the following values are
printed to the log:

Debug: 2915 2020 arc.c:894 arc_save_context(): Get core register regnum=0,
name=r0, value=0x0000000
...
Debug: 2947 2020 arc.c:894 arc_save_context(): Get core register regnum=60,
name=lp_count, value=0x900002d8
Debug: 2948 2020 arc.c:894 arc_save_context(): Get core register regnum=63,
name=pcl, value=0xffffffff
Debug: 2949 2020 arc.c:909 arc_save_context(): Get aux register regnum=64,
name=pc, value=0x900000b4
Debug: 2950 2020 arc.c:909 arc_save_context(): Get aux register regnum=65,
name=lp_start, value=0x900000bc
Debug: 2951 2020 arc.c:909 arc_save_context(): Get aux register regnum=66,
name=lp_end, value=0x00080801
Debug: 2952 2020 arc.c:909 arc_save_context(): Get aux register regnum=67,
name=status32, value=0xffffffff

After the change, the register contents make much more sense:

Debug: 2923 3934 arc.c:889 arc_save_context(): Get core register regnum=0,
name=r0, value=0x00000000
...
Debug: 2955 3934 arc.c:889 arc_save_context(): Get core register regnum=60,
name=lp_count, value=0x00000000
Debug: 2956 3934 arc.c:889 arc_save_context(): Get core register regnum=63,
name=pcl, value=0x900002d8
Debug: 2957 3934 arc.c:903 arc_save_context(): Get aux register regnum=64,
name=pc, value=0x900002da
Debug: 2958 3934 arc.c:903 arc_save_context(): Get aux register regnum=65,
name=lp_start, value=0x900000b4
Debug: 2959 3934 arc.c:903 arc_save_context(): Get aux register regnum=66,
name=lp_end, value=0x900000bc
Debug: 2960 3934 arc.c:903 arc_save_context(): Get aux register regnum=67,
name=status32, value=0x00080801

While at it, simplify a couple of expressions.

Change-Id: I8f2d79404707fbac4503af45b393ea73f91e6beb
Signed-off-by: Artemiy Volkov <artemiy@synopsys.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7765
Tested-by: jenkins
Reviewed-by: Evgeniy Didin <didin@synopsys.com>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>