At least in some ATTRIBUTE_COLD code, mach_read_from_8() could be
invoked as a function call that could be as simple as wrapping
one or two instructions. Let us declare __attribute__((always_inline))
on those memory accessor functions that operate on 1, 2, 4, or 8 bytes
and are therefore likely to translate into few instructions, such as
mov;bswap or movbe on x86.
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