Fix search for LLVM to prefer LLVM_ROOT if specified

[TimoWilken]

If the system has LLVM 13, but we want to build with LLVM 12, this way we can set LLVM_ROOT or LLVM_DIR to force v12 to be used.

As far as I can tell, this works for me. In an slc8-gpu container, libgandiva is not linked against the system LLVM 13:

[root@d91eb64d9790 ~]# ldd sw/slc8_x86-64/arrow/v5.0.0-alice2-local1/lib/libgandiva.so
sw/slc8_x86-64/arrow/v5.0.0-alice2-local1/lib/libgandiva.so: /lib64/libstdc++.so.6: version `GLIBCXX_3.4.26' not found (required by sw/slc8_x86-64/arrow/v5.0.0-alice2-local1/lib/libgandiva.so)
        linux-vdso.so.1 (0x00007ffc26199000)
        libarrow.so.500 => not found
        libcrypto.so.1.0.0 => /root/sw/slc8_x86-64/OpenSSL/v1.0.2o-6/lib/libcrypto.so.1.0.0 (0x00007f7e8341e000)
        libssl.so.1.0.0 => /root/sw/slc8_x86-64/OpenSSL/v1.0.2o-6/lib/libssl.so.1.0.0 (0x00007f7e8095d000)
        libutf8proc.so.2 => /root/sw/slc8_x86-64/utf8proc/v2.6.1-5/lib/libutf8proc.so.2 (0x00007f7e80908000)
        libdl.so.2 => /lib64/libdl.so.2 (0x00007f7e80704000)
        librt.so.1 => /lib64/librt.so.1 (0x00007f7e804fc000)
        libpthread.so.0 => /lib64/libpthread.so.0 (0x00007f7e802dc000)
        libz.so.1 => /root/sw/slc8_x86-64/zlib/v1.2.8-5/lib/libz.so.1 (0x00007f7e83403000)
        libtinfo.so.6 => /lib64/libtinfo.so.6 (0x00007f7e800af000)
        libstdc++.so.6 => /lib64/libstdc++.so.6 (0x00007f7e7fd1a000)
        libm.so.6 => /lib64/libm.so.6 (0x00007f7e7f998000)
        libgcc_s.so.1 => /lib64/libgcc_s.so.1 (0x00007f7e7f780000)
        libc.so.6 => /lib64/libc.so.6 (0x00007f7e7f3bb000)
        /lib64/ld-linux-x86-64.so.2 (0x00007f7e833c0000)

[TimoWilken]

(Force-pushed to fix formatting only.)

[pzhristov]

The test on epn243 is OK.