perf build x86: Fix SC2034 error in syscalltbl.sh

perf build x86: Fix SC2034 error in syscalltbl.sh

Change the unused var in 'arch/x86/entry/syscalls/syscalltbl.sh' to '_'
when reading from '$sorted_table'. This change allows the script to pass
tests of ShellCheck before and after version 0.7.2 at the same time.

When building in arch x86, syscalltbl.sh got a ShellCheck warning, which
makes compilation error:

    In arch/x86/entry/syscalls/syscalltbl.sh line 27:
    while read nr _abi name entry _compat; do
                  ^-^ SC2034: abi appears unused.
                  Verify use (or export if used externally).
                                  ^----^ SC2034: compat appears unused.
                               Verify use (or export if used externally).

The script reads unused param abi and compat. It uses format '_xxx' to
indicate dummy vars, which won't work properly when ShellCheck <= 0.7.2.

According to SC2034, the more general way of writing is to use directly
'_' to indicate discarding vars. 'entry' is also replaced by '_' because
it just happens to be defined in emit function, otherwise it will lead
to some misunderstandings.

Link: https://www.shellcheck.net/wiki/SC2034
Signed-off-by: Haoze Xie <royenheart@gmail.com>
Signed-off-by: Yuan Tan <tanyuan@tinylab.org>
Link: https://lore.kernel.org/r/2143cab4cd8468c88860f4e5e382d0e6b4d89ac9.1720372178.git.royenheart@gmail.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>