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ARROW-7554: [C++] Add support for building on FreeBSD

I just confirmed that we can build without any CMake options.
I didn't confirm that our test suite is passed.

We can't build jemalloc with --without-libdl because jemalloc requires
lazy-lock feature on FreeBSD and lazy-lock feature requires libdl.

FreeBSD has /usr/include/machine/endian.h instead of
/usr/include/endian.h.

FreeBSD doesn't have MREMAP_MAYMOVE.

Closes #6168 from kou/cpp-make-buildabile-on-freebsd and squashes the following commits:

8e821bc49 <Sutou Kouhei> Disable jemalloc explicitly on Windows
7571d2f21 <Sutou Kouhei> ARROW-7554:  Add support for building on FreeBSD

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>