libstdc++: Use explicit cast to unsigned in std::rotr and std::rotl

libstdc++: Use explicit cast to unsigned in std::rotr and std::rotl

This suppresses some -Wsign-conversion warnings from Clang when
compiling with -Wsystem-headers.

libstdc++-v3/ChangeLog:

	* include/std/bit (__rotl, __rotr): Use static_cast for
	conversion from int to unsigned.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>