libsnapshot: Use words for xor ops

Use words instead of doing xor byte-by-byte for the entire buffer.
Profiling this with unittest, I can see the xor ops is much faster.

Also handle the word size appropriately for 32bit and 64bit platforms.

simpleperf shows that ProcessXorOp() is atleast 30% faster.
Similar improvement is seen for ProcessXorData().

Test: snapuserd_test
Bug: 369905394
Change-Id: I0bd8586f7fc1bf184f19320667b8195b07f9cdf2
Signed-off-by: Sandeep Dhavale <dhavale@google.com>