This is needed to avoid the following error when using
-fsanitize=undefined with gcc:
lib/x86/adler32_impl.h:214:2: runtime error: signed integer overflow:
1951294680 + 1956941400 cannot be represented in type 'int'
Note that this isn't seen when using -fsanitize=undefined with clang.
Old compilers don't have unsigned vector types, so work around that.
Now that we detect CPU features on 32-bit x86, allow the SSE2
implementation of Adler-32 to be selected at runtime based on the
presence of the SSE2 feature.
Move the x86 and ARM-specific code into their own directories to prevent
it from cluttering up the main library. This will make it a bit easier
to add new architecture-specific code.
But to avoid complicating things too much for people who aren't using
the provided Makefile, we still just compile all .c files for all
architectures (irrelevant ones end up #ifdef'ed out), and the headers
are included explicitly for each architecture so that an
architecture-specific include path isn't needed. So, now people just
need to compile both lib/*.c and lib/*/*.c instead of only lib/*.c.