Due to a language flaw in C, left-shifts on signed integers are
undefined for negative numbers. This makes them all but useless. Cast to
the unsigned type, left-shift, and cast back (casts are defined to wrap)
to silence UBSan.

Change-Id: I8fbe739aee1c99cf553462b675863e6d68c2b302
Reviewed-on: https://boringssl-review.googlesource.com/c/34446
Reviewed-by: Adam Langley <agl@google.com>