[gdb/contrib] Factor out grep_or and sed_or in spellcheck.sh

[gdb/contrib] Factor out grep_or and sed_or in spellcheck.sh

While trying to add more separators here:
...
 # Separators: space, slash, tab.
 grep_separator=" |/|	"
 sed_separator=" \|/\|\t"
...
I mistakingly used "|" instead of "\|" in sed_separator.

Factor out new variables grep_or and sed_or, and construct the grep_separator
and sed_separator variables by joining the elements of a list using grep_or
and sed_or.

Verified with shellcheck, and tested by rerunning on x86_64-linux.

Reviewed-By: Alexandra Petlanova Hajkova <ahajkova@redhat.com>