bash: fix wrong exit status offset
In Linux，8 bits of the return code and 8 bits of the number of
the killing signal are mixed into a single value on the exit code,
so the exit status offset should be 8. But the autoconf checker
can not determine it while cross compiling, and then it is set to
the default value 0, which will cause generating the wrong exit
code if program exit with an error code.
(From OE-Core rev:
e2dea46607a24620d6d2c250efc9b2e95bfd5ad8)
Signed-off-by: Rui Wang <rui.wang@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Armin Kuster <akuster808@gmail.com>
