[gdb/testsuite] Add have_host_locale

[gdb/testsuite] Add have_host_locale

With test-case gdb.tui/pr30056.exp, I run into:
...
sh: warning: setlocale: LC_ALL: cannot change locale (C.UTF-8)^M
...
and then subsequently into:
...
WARNING: timeout in accept_gdb_output
FAIL: gdb.tui/pr30056.exp: Control-C
...

This is on a CentOS 7 distro for powerpc64le.

Either it has no C.UTF-8 support, or it's not installed:
...
$ locale -a | grep ^C
C
$
...

Fix this by:
- adding a new proc have_host_locale, and
- using it in all test-cases using setenv LC_ALL.

Tested on powerpc64le-linux and x86_64-linux.