[PATCH, SEH] Add hook for location of target-unwind directives

[Richard Henderson <rth at redhat dot com>]

The IA-64 and ARM target specific unwind directives go in front
of the instruction to which they apply, but the x64 SEH unwind
directives go after the instruction (which makes a lot more
sense on an arch with variable-length insns).

This hook allows the middle-end to emit the directive in the
right place for each.

Tested with x86_64-w64-mingw32 (plus other patches), cross to
ia64-linux and arm-linux-eabi, and x86_64-linux sanity check.

Committed.


r~

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