[Patch][ARM] Add -mbranch-cost option, and update a few tests

[Christophe Lyon <christophe dot lyon at linaro dot org>]

Hi,

After Jakub's suggestion in PR82120 and PR81184, the attached patch
adds the -mbranch-cost option to the ARM target. My understanding
is that it's intended to be used internally for testing and does not
require user-facing documentation.

I have updated a few tests, validation on aarch64 & arm targets shows
no regression,
and a few improvements when targeting cortex-a5 or cortex-m3:
gcc.dg/tree-ssa/reassoc-3[3456].c now pass.

That being said, I'm not sure about the other targets for which I
changed the condition,
and I am also concerned by the fact that it has no impact on
gcc.dg/pr21643.c and gcc.dg/tree-ssa/phi-opt-11.c (PR81184).

Should I restrict my patch to the only tests where it has an impact
(gcc.dg/tree-ssa/reassoc-3[3456].c) ?

Thanks,

Christophe

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