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.:
	./scripts/checkpatch.pl --git 81c2f059
	WARNING: 'ment' may be misspelled - perhaps 'meant'?
	#36: FILE: MAINTAINERS:14360:
	+M:     Clément Léger <clement.leger@bootlin.com>
	            ^^^^

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

Link: https://lkml.kernel.org/r/20250616-b4-checkpatch-upstream-v2-1-5600ce4a3b43@foss.st.com


Signed-off-by: Antonio Borneo <antonio.borneo@foss.st.com>
Signed-off-by: Clément Le Goffic <clement.legoffic@foss.st.com>
Cc: Andy Whitcroft <apw@canonical.com>
Cc: Dwaipayan Ray <dwaipayanray1@gmail.com>
Cc: Joe Perches <joe@perches.com>
Cc: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>