[C++] current_function_is_thunk

[Richard Henderson]

Alpha is currently broken with the v3 ABI.

This is because we assume that if current_function_is_thunk
is set, then we will be calling ASM_OUTPUT_MI_THUNK.  When
this doesn't happen, we end up miscompiling the real function.

I guess there are several choices:

(1) Disable ASM_OUTPUT_MI_THUNK in alpha.h.

(2) Define a new flag that means exactly "assemble_start_function,
    you are getting ready for ASM_OUTPUT_MI_THUNK".

(3) Figure out what sorts of things the v3 ABI needs different
    and define a new ASM_OUTPUT_MI_V3_THUNK.



r~