[libc][math] Improve the performance of subtraction for UInt<>.

Use built-in sub_with_borrow to improve the performance of subtraction
for UInt<>.

Microbenchmark: https://quick-bench.com/q/UEg6Z6VK4PP2yLBtTi9C0cL9xpY

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D138506