libstdc++: Use static_cast on OpenBSD ctype_base masks

Commit r17-3313-g15419a00a29342 fixed the build on newer OpenBSD
versions.  However, on platforms where char is signed, the mask
constants may be implicitly converted to negative values, causing
overflow/signedness change warnings.  Add static_cast<mask>(...) to
make the conversion explicit and silence the warnings.

libstdc++-v3/ChangeLog:

	* config/os/bsd/openbsd/ctype_base.h: Use static_cast when
	setting the masks.

Signed-off-by: Pietro Monteiro <pietro@sociotechnical.xyz>
(cherry picked from commit 18f299849d3dd151b62e2214dfebaa5ecdebe9b5)