JIT: handle general indir case for GT_JMP address

JIT: handle general indir case for GT_JMP address

Some upcoming changes to reduce tiering overhead require that directly
invoked virtual methods be called indirectly via their slot, so that the
method body can be updated and callers patched up by patching the method
table slot.

Existing code for x64 implicitly assumes that a GT_JMP indirect target address
is near enough to the call site that a 32 bit RIP-relative displacement will
work. We can ensure this is true by always generating a reloc (and hence
potentially a jump stub) -- unless the target happens to fit in 32 bits and
so can be addressed absolutely.