Headers: mark arm_acle.h with extern "C"

Although the functions are marked as always_inline, the compiler with which they
are used may not honour the extended attributes and emit them as functions.  In
such a case, indicate that they should have extern "C" linkage and should not be
mangled in C++ style if used within C++.
Headers: conditionalise more declarations

Protect MMX specific declarations under a __MMX__ guard.  This header can be
included on non-x86 architectures (e.g. ARM) which do not support the MMX ISA.
Use the preprocessor to prevent these declarations from being processed.