Fix alpha-linux unwind info

[Richard Henderson <rth at redhat dot com>]

See http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49688

It turns out that given both .ent/.end (aka "legacy unwind") directives
and CFI directives, we were generating double .eh_frame entries.  Given
that the legacy directives are not able to represent personality data,
this lead to a bit of random failure as the binary search selected one
or the other of the two .eh_frame entries with equal start addresses.

The existing test for the existance of .eh_frame did not work because
we have not yet called cfi_finish.

Exposing the dw2gencfi variable seems slightly hacky, but no worse
than any other solution I could think of.

Ok?


r~

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