[m-rev.] for review: Only assume structure assignment conflicts with global registers on x86.

[Julien Fischer]

On Thu, 15 Oct 2020, Peter Wang wrote:

> configure.ac:
>    As above. The result of the "structure assignment conflicts with
>    global registers" test was hard coded to fail on gcc >= 3, but the
>    issue it avoids possibly only occurs on x86. I cannot reproduce it
>    on x86-64 (with gcc 8.3.0) but I *can* reproduce it on x86.

I think it was only ever an issue on x86 due to the lack of registers
on that platform.  (That's certainly the impression I get from this revision
history of runtime/mercury_reg_workarounds.h etc.)

Julien.