build: Fix HAVE_ATTRIBUTE_PACKED and HAVE_PRAGMA_PACK checks
build: Fix HAVE_ATTRIBUTE_PACKED and HAVE_PRAGMA_PACK checks

C++14 standard has a std::static_assert, so let's rename the struct used
in HAVE_ATTRIBUTE_PACKED and HAVE_PRAGMA_PACK checks.

Fixed by @ergo720.

libcpu/libcpu,libcpu/libcpu,libcpu/libcpu,libcpu/libcpu