Add support for the AMD64 syntax [symbol wrt rip]. This is distinct from

Add support for the AMD64 syntax [symbol wrt rip].  This is distinct from
[rip+symbol] in that the latter adds the symbol offset to rip, whereas the
former is the same as [symbol] but uses rip-relative addressing.  This is
a minor overload of the WRT operator, but reads well and shouldn't conflict
with the use of WRT against sections.

Doing this currently adds a bit of overhead to all effective addresses in
64-bit mode (a $ symbol reference).  This is the cleanest approach I could
figure out; a time/space trade could be made later, such as prescanning for
RIP usage before allocating the symbol.

svn path=/trunk/yasm/; revision=1032