util/bitset: Make C++ wrapper trivially constructible.

util/bitset: Make C++ wrapper trivially constructible.

In order to fix a build failure on compilers not implementing
unrestricted unions, which is a C++11 feature.

v2: Provide signed integer comparison and assignment operators instead
    of BITSET_WORD ones to avoid spurious ambiguity warnings on
    comparisons with a signed integer literal.

Fixes: ba79a90fb52e1e81fb "glsl: Switch ast_type_qualifier to a 128-bit bitset."
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=105238
Tested-by: Roland Scheidegger <sroland@vmware.com>
Tested-By: George Kyriazis <george.kyriazis@intel.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>