[llvm lib] Fix endian bug in #189098 (#189778)
The bug was added in #189098. Both functions that I added/changed
assumed little-endian host memory layout when reading/writing
non-power-of-two byte sizes (3, 5, 6, 7). On big-endian hosts (SPARC,
s390x), memcpy copies bytes into the MSB end of a uint64_t, leaving the
value in the wrong bit position. Fix by offsetting the memcpy/write
pointer to always target the LSB bytes.
