[gdb/testsuite] Fix false negative in have_host_locale

[gdb/testsuite] Fix false negative in have_host_locale

In test-case gdb.tui/pr30056.exp we check for:
...
require {have_host_locale C.UTF-8}
...

The "C.UTF-8" is normalized by have_host_locale to "c.utf8", before trying to
find it in the list returned by host_locales.

On my development platform, "locale -a" lists C.utf8, which is normalized to
"c.utf8" by host_locales, so there's a match and have_host_locale returns true.

On another platform however, "locale -a" lists C.UTF-8, which is normalized to
"c.utf-8" by host_locales, so there's no match and have_host_locale returns false.

Fix this by also dropping the dash in host_locales.

Tested on x86_64-linux.