Signed flag is now forwarded to the backends.

Forwarding the "signed" property allows to specialize operations
implementation, which may be desired for some architectures. This also
eliminates the formally implementation-defined result of unsigned-to-
target type.

Also for x86 implementations removed compiler barriers since the
compilers already treat intrinsics/asm blocks as ones.
Fixed compilation.
Implemented PorwerPC backend.
Implemented gcc SPARCv9 backend.

The backend also adds support for 64-bit atomics.