Moved the variable VGOFF_(m_eip) from x86, and moved it (and its

Moved the variable VGOFF_(m_eip) from x86, and moved it (and its
computation) into the generic core, because it's needed by the dispatch
loop of all archs.  Also renamed it VG_(instr_ptr_offset), which is
clearer.  Since it was the last use of the VGOFF_ prefix, I removed that
prefix.

Also cleaned up the ARM stubs slightly, by removing some of the copied,
commented-out x86 code which has since fallen out of date with the real
x86 code and was thus misleading.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@3123