Introduce compiler.h header file

Introduce compiler.h header file

This moves compiler related macros from qemu-common.h to compiler.h.

The reason for this change is that there are simple header files that
depend only on the compiler macros, so including qemu-common.h is overkill.

Besides, qemu-common.h is bloated and will benefit from some splitting.

Please, also note that the QEMU_BUILD_BUG_ON() macro is being fixed to
not use double underscores as a prefix and the license text was added
by Vassili Karpov (malc), who is one of the authors of the new file.

Signed-off-by: Luiz Capitulino <lcapitulino@gmail.com>