begin refactoring load address computations in dynamic linker

begin refactoring load address computations in dynamic linker

for ordinary ELF with fixed segment displacements, load address
computation is simply adding the base load address. but for FDPIC,
each segment has its own load address, and virtual addresses need to
be adjusted according to the segment they fall in. abstracting this
computation is the first step to making the dynamic linker ready for
FDPIC.

for this first commit, a macro is used rather than a function in order
to facilitate correctness checking. I have verified that the generated
code does not change on my i386 build.