gdb/configure: Fix POSIX non-compliance

My recent GDB commit:

    commit 4b42385c470c5f72f158f382f4d9c36f927aa84f
    Author: Guinevere Larsen <guinevere@redhat.com>
    Date:   Wed Feb 12 08:25:46 2025 -0300
    gdb: Make dwarf support optional at compile time

Introduced a change that made the configure script not POSIX compliant,
by using fallthrough in some case statements. This commit reworks that
part of the change to only use if statements, so that no code is
duplicated but things remain POSIX compliant.

Reviewed-by: Sam James <sam@gentoo.org>
Approved-By: Tom Tromey <tom@tromey.com>