1.0.1 fails to build on FreeBSD

Regressed by 7438638586ae. Found [downstream](https://www.freshports.org/devel/highway). On FreeBSD libatomic is not present because the default toolchain is LLVM-based (Clang, LLD, libc++, compiler-rt).



```

$ cmake -DBUILD_TESTING=off -G Ninja -B /tmp/highway_build

-- The C compiler identification is Clang 14.0.5

-- The CXX compiler identification is Clang 14.0.5

-- Detecting C compiler ABI info

-- Detecting C compiler ABI info - done

-- Check for working C compiler: /usr/bin/cc - skipped

-- Detecting C compile features

-- Detecting C compile features - done

-- Detecting CXX compiler ABI info

-- Detecting CXX compiler ABI info - done

-- Check for working CXX compiler: /usr/bin/c++ - skipped

-- Detecting CXX compile features

-- Detecting CXX compile features - done

-- Performing Test HWY_EMSCRIPTEN

-- Performing Test HWY_EMSCRIPTEN - Failed

-- Performing Test HWY_RISCV

-- Performing Test HWY_RISCV - Failed

-- Configuring done

-- Generating done

-- Build files have been written to: /tmp/highway_build



$ cmake --build /tmp/highway_build

[22/35] Linking CXX executable hwy_list_targets

ld: error: unable to find library -latomic

c++: error: linker command failed with exit code 1 (use -v to see invocation)

[30/35] Linking CXX executable examples/hwy_benchmark

FAILED: examples/hwy_benchmark

: && /usr/bin/c++ -O2 -g -DNDEBUG -fPIE -pie CMakeFiles/hwy_benchmark.dir/hwy/examples/benchmark.cc.o -o examples/hwy_benchmark  libhwy.a  -latomic && :

ld: error: unable to find library -latomic

c++: error: linker command failed with exit code 1 (use -v to see invocation)

[34/35] Building CXX object CMakeFiles/hwy_contrib.dir/hwy/contrib/sort/vqsort_u16d.cc.o

ninja: build stopped: subcommand failed.

```

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