Fix jemalloc compatibility with LLVM libunwind

[georgthegreat]

jemalloc provides support for two different libunwind flavors: the original HP libunwind and the one coming with gcc / g++ / libstdc++.

The latter is identified by JEMALLOC_PROF_LIBGCC and uses _Unwind_Backtrace method instead of unw_backtrace.
At the time ClickHouse uses LLVM libunwind which follows libgcc's way of backtracing.

ClickHouse has to provide unw_backtrace method by the means of commit 8e2b31e.

While this PR does not allow complete removal of the patch (as ClickHouse itself uses unw_backtrace directly), it definitely sorts some things out.

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• Not for changelog (changelog entry is not required)

[georgthegreat]

@alexey-milovidov, @nikitamikhaylov, could you merge this?

[alexey-milovidov]

Functional test has failed: https://s3.amazonaws.com/clickhouse-test-reports/37078/6d28b226878f935505edef2606c860da3b58bc9b/stateless_tests__thread__actions__[3/3].html

Obviously, it is not related to the changes in this PR but I will use this fact to highlight the principle that pull requests should be 100% green, regardless. We need to investigate and fix the test and only then merge this PR.

[alexey-milovidov]

@Mergifyio update

[mergify]

update

✅ Branch has been successfully updated

[alexey-milovidov]

I merged prematurely, but the code is non-obvious, it needs a comment.

It's literally impossible to understand, why

if (USE_UNWIND)
    target_compile_definitions (_jemalloc PRIVATE -DJEMALLOC_PROF_LIBGCC=1)

when libunwind is enabled we use "libgcc" for profiling in jemalloc.
It will introduce "WTF" when anyone will read the code.

Need to copy-paste your description from the pull request to the code.