Use --icf=all instead of --icf=safe

With --icf=safe, functions whose address is taken aren't folded.
Folding could change the results of function pointer comparisons.

However, on x86_64 with pie, --icf=safe disables almost all folding,
for technical reasons. Since using --icf=all will save 7 MB on x86_64
and all tests seem to pass with it, switch to --icf=all. If we end up
having a function where we absolutely need equality, we can pass
--keep-unique,<symbol-name> for that symbol.

And if this turns out to be some maintenance burden, we can switch
back to --icf=safe.

Change the flag on all platforms where we use gold, not just x86_64, for
consistency so that if we end up having problems with this, we notice
it faster.

BUG=76280,492177

Review URL: https://codereview.chromium.org/1153143004

Cr-Commit-Position: refs/heads/master@{#331474}

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