[gdb/contrib] Fix spellcheck.sh for bash < 5.1

Since commit 5cb0406bb64 ("[gdb/contrib] Handle capitalized words in
spellcheck.sh"), spellcheck.sh uses '${pat@u}' which is available starting
bash 5.1, and consequently the script breaks with bash 4.4.

Fix this by checking for the bash version, and using an alternative
implementation for bash < 5.1.

Tested on x86_64-linux.