shared: add helper function to add and check for overflow

shared: add helper function to add and check for overflow

Use _builtin_uaddll_overflow/_builtin_uaddl_overflow when available,
abstracting the type to use it with uint64_t.

Otherwise fallback to the implementation as added in 67466f2 ("Prevent
offset + size overflow.").

This also adds the tests for this new helper in the testsuite.