[gdb/testsuite] Skip stabs board in make-check-all.sh

[gdb/testsuite] Skip stabs board in make-check-all.sh

I ran make-check-all.sh with gdb.linespec/explicit.exp, and the only problems
were found with target board stabs.

With target board unix the test-case runs in two seconds, but with target
board stabs it takes 12 seconds due to a timeout.

Stabs support in gdb has been unmaintained for a while, and there's an ongoing
discussion to deprecate and remove it (PR symtab/31210).

It seems unnecessary to excercise this unmaintained feature in
make-check-all.sh, so drop it.

Tested on x86_64-linux.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31210