New backend interface - assembler internals #1

New backend interface - assembler internals #1

This provides the first pass of assembler internals for a new, richer,
backend interface.  It also reduces the amount of data carried in
function parameters inside the assembler.

The new interface is in the form of a structure, which will contain
substantially more information that the previous interface.  This will
allow things like ELF GOT32X and Mach-O X86_64_RELOC_BRANCH
relocations to be reliably emitted.

This provides the initial set of structures.  Future additions should
include:

1. Pass down the base symbol, if any.
2. Segments should be a structure rather than a number, and the
   horrible hack of adding one for a segment reference should be
   removed (OUT_SEGMENT replaces it.)

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>