Add function link_elf_get_gp(), specific to ia64 for now, to get

Add function link_elf_get_gp(), specific to ia64 for now, to get
the DT_PLTGOT value. On ia64 this is the value of GP. We need this
to construct function descriptors, but the elf file structure is
not exported to MD code.

Note that the name of the function is based on the meaning that
DT_PLTGOT has on ia64. This may differ on other architectures. As
such, link_elf_get_gp() has a high level of MD to it. Renaming the
function to describe what DT_* value is returned makes it generic,
but also makes the MD code less clear and if we only need this on
ia64, then a general name for a specific function doesn't help.

In short: I don't know what is "right" at this time, so I'll go
with what I have.