[libc] fix shifting exact multiples of 64 in uint

The internal uint class had a bug introduced recently when optimizing
the shift routines. When calculating the value of a block, it would
shift an adjacent block by 64 - the shift amount. If the shift amount
was 0, this would be a shift of 64, which is undefined for a 64 bit
integer. This patch fixes this by adding a conditional to catch this
case.

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D140070