RFC: BB duplication code

[Joern Rennecke]

> In the case of JUMP_TABLES_IN_TEXT_SECTION, yes, you absolutely cannot
> share the table.  This is used by targets that use pc-relative addressing
> from the jump instruction to the table.  VAX is the most extreme example
> in that the entire table is an argument to the instruction.

This is target-dependent.  Some targets just put the jump table in the text
section because the linker can't handle cross-section differences, and
they use differences (either always or for pic / PIC).