checkpatch: use utf-8 match for spell checking

The current code that checks for misspelling verifies, in a more
complex regex, if $rawline matches [^\w]($misspellings)[^\w]

Being $rawline a byte-string, a utf-8 character in $rawline can
match the non-word-char [^\w].
E.g.:
	./script/checkpatch.pl --git 81c2f05
	WARNING: 'ment' may be misspelled - perhaps 'meant'?
	torvalds#36: FILE: MAINTAINERS:14360:
	+M:     Clément Léger <clement.leger@bootlin.com>
	            ^^^^

Use a utf-8 version of $rawline for spell checking.

Signed-off-by: Antonio Borneo <antonio.borneo@foss.st.com>
Reported-by: Clément Le Goffic <clement.legoffic@foss.st.com>

scripts/checkpatch.pl

@@ -3477,7 +3477,8 @@ sub process {
 # Check for various typo / spelling mistakes
 		if (defined($misspellings) &&
 		    ($in_commit_log || $line =~ /^(?:\+|Subject:)/i)) {
-			while ($rawline =~ /(?:^|[^\w\-'`])($misspellings)(?:[^\w\-'`]|$)/gi) {
+			my $rawline_utf8 = decode("utf8", $rawline);
+			while ($rawline_utf8 =~ /(?:^|[^\w\-'`])($misspellings)(?:[^\w\-'`]|$)/gi) {
 				my $typo = $1;
 				my $blank = copy_spacing($rawline);
 				my $ptr = substr($blank, 0, $-[1]) . "^" x length($typo);