Avoid use of Vietnamese UTF-8 locale on Solaris

On OmniOS r151042 (based on Solaris) the 'strfxm_l' function segfaults
when:

- LC_COLLATE_MASK is set to 'vi_VN.UTF-8'
- the input contains "\xA8"

Other locales and other characters appear to be unaffected.

This commit causes 'loc_tools.pl' to not return this locale as being
acceptable when asked for a list of them.  Thus, our automated locale
testing will avoid it.

A Perl program can still switch into it explicitly.
This issue is tracked by #20578, which includes a C-only reproducer not
involving Perl.

Author: khwilliamson

t/loc_tools.pl

@@ -17,7 +17,8 @@
 
 my %known_bad_locales = (   # XXX eventually will need version info if and
                             # when these get fixed.
-                        );
+    solaris => [ 'vi_VN.UTF-8', ],  # Use of U+A8 segfaults: GH #20578
+);
 
 eval { require POSIX; import POSIX 'locale_h'; };
 my $has_locale_h = ! $@;