[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.

gdb/testsuite/gdb.ada/non-ascii-latin-1.exp

@@ -18,6 +18,7 @@
 load_lib "ada.exp"
 
 require allow_ada_tests
+require {have_host_locale C.UTF-8}
 
 # Enable basic use of UTF-8.  LC_ALL gets reset for each testfile.  We
 # want this despite the program itself using Latin-1, as this test is

gdb/testsuite/gdb.ada/non-ascii-latin-3.exp

@@ -18,6 +18,7 @@
 load_lib "ada.exp"
 
 require allow_ada_tests
+require {have_host_locale C.UTF-8}
 
 # Enable basic use of UTF-8.  LC_ALL gets reset for each testfile.  We
 # want this despite the program itself using Latin-1, as this test is

gdb/testsuite/gdb.ada/non-ascii-utf-8.exp

@@ -18,6 +18,7 @@
 load_lib "ada.exp"
 
 require allow_ada_tests
+require {have_host_locale C.UTF-8}
 
 # Enable basic use of UTF-8.  LC_ALL gets reset for each testfile.
 setenv LC_ALL C.UTF-8

gdb/testsuite/gdb.base/utf8-identifiers.exp

@@ -25,6 +25,7 @@ if { [is_c_compiler_gcc] } {
     # Gcc fully supports fextended-identifiers starting GCC 5.
     require {expr [gcc_major_version] >= 5}
 }
+require {have_host_locale C.UTF-8}
 
 standard_testfile
 

gdb/testsuite/gdb.rust/unicode.exp

@@ -18,6 +18,7 @@
 load_lib rust-support.exp
 require allow_rust_tests
 require {can_compile rust}
+require {have_host_locale C.UTF-8}
 
 # Non-ASCII identifiers were allowed starting in 1.53.
 require {rust_at_least 1.53}

gdb/testsuite/gdb.tui/pr30056.exp

@@ -18,6 +18,7 @@
 # This PR is fixed in the in-repo copy of readline.  System readline may or
 # may not be fixed, so skip this test-case.
 require !with_system_readline
+require {have_host_locale C.UTF-8}
 
 tuiterm_env
 

gdb/testsuite/lib/gdb.exp

@@ -9794,5 +9794,41 @@ proc lappend_include_file { flags file } {
     }
 }
 
+# Return a list of supported host locales.
+
+gdb_caching_proc host_locales { } {
+    set result [remote_exec host "locale -a"]
+    set status [lindex $result 0]
+    set output [lindex $result 1]
+
+    if { $status != 0 } {
+	return {}
+    }
+
+    # Split into list.
+    set output [string trim $output]
+    set l [split $output \n]
+
+    # Trim items.
+    set l [lmap v $l { string trim $v }]
+
+    # Normalize items to lower-case.
+    set l [lmap v $l { string tolower $v }]
+
+    return $l
+}
+
+# Return 1 if host locale LOCALE is supported.
+
+proc have_host_locale { locale } {
+    # Normalize to lower-case.
+    set locale [string tolower $locale]
+    # Normalize to without dash.
+    set locale [string map { "-" "" } $locale]
+
+    set idx [lsearch [host_locales] $locale]
+    return [expr $idx != -1]
+}
+
 # Always load compatibility stuff.
 load_lib future.exp