[PATCH] Detect whether target can use -fprofile-update=atomic

[Martin Liška]

Hello.

As reported in [1], m68k has been broken since I installed the patch. Reason is that the target
does not support atomic operations (add, or) for a mode of gcov_type. Because of that, we see
an undefined symbols.

Proper fix contains of 2 parts:
a) compiler emission must verify that -fprofile-update=atomic is doable for a given target; it's done
via a new function can_generate_atomic_builtin
b) libgcc must detect whether __atomic_fetch_add_x can be expanded on the target; that requires configure
support and if the target is not capable to expand these, we must conditionally remove all gcov_.*profiler_atomic
functions from libgcov.a.

Andreas reported that it fixes the test-case mentioned in the PR and I tested that on -march=i386.
Apart from that I've been doing bootstrap on x86_64-linux-gnu.

Ready after it finishes?
Martin

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58306#c30
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