include/qemu/bswap.h: Use __builtin_memcpy() in accessor functions

In the accessor functions ld*_he_p() and st*_he_p() we use memcpy()
to perform a load or store to a pointer which might not be aligned
for the size of the type. We rely on the compiler to optimize this
memcpy() into an efficient load or store instruction where possible.
This is required for good performance, but at the moment it is also
required for correct operation, because some users of these functions
require that the access is atomic if the pointer is aligned, which
will only be the case if the compiler has optimized out the memcpy().
(The particular example where we discovered this is the virtio
vring_avail_idx() which calls virtio_lduw_phys_cached() which
eventually ends up calling lduw_he_p().)

Unfortunately some compile environments, such as the fortify-source
setup used in Alpine Linux, define memcpy() to a wrapper function
in a way that inhibits this compiler optimization.

The correct long-term fix here is to add a set of functions for
doing atomic accesses into AddressSpaces (and to other relevant
families of accessor functions like the virtio_*_phys_cached()
ones), and make sure that callsites which want atomic behaviour
use the correct functions.

In the meantime, switch to using __builtin_memcpy() in the
bswap.h accessor functions. This will make us robust against things
like this fortify library in the short term. In the longer term
it will mean that we don't end up with these functions being really
badly-performing even if the semantics of the out-of-line memcpy()
are correct.

Reported-by: Fernando Casas Schössow <casasfernando@outlook.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20190318112938.8298-1-peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>