[libc] Maintain proper alignment for the hermetic tests malloc

We use a bump pointer to implement malloc for the hermetic tests.
Currently, we bump the pointer up by any amount. This means that calling
`malloc(1)` will misalign the buffer so any following `malloc(8)`
accesses will not be aligned. This causes problems in architectures
which require alignment.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D149863