qemu/compiler: Wrap __attribute__((flatten)) in a macro

qemu/compiler: Wrap __attribute__((flatten)) in a macro

Older versions of Clang (before 3.5) and GCC (before 4.1) do not
support the "__attribute__((flatten))" yet. We don't care about
such old versions of GCC anymore, but since Clang 3.4 is still
used in EPEL for RHEL7 / CentOS 7, we should not use this attribute
directly but with a wrapper macro instead.

Reviewed-by: Alex Bennée <[email protected]>
Reviewed-by: Richard Henderson <[email protected]>
Signed-off-by: Thomas Huth <[email protected]>