Add FindVc.cmake to AtlasLCG, master branch (2022.02.25.)

Added FindVc.cmake to AtlasLCG, and made sure that AthenaExternals would make use of it, mainly to make ROOT functional in the LCG nightlies.

Pinging @mnowak, @elmsheus and @simmons.

Note that calling find_package(Vc) ends up printing quite a lot of crap...

...
-- Detected Compiler: GCC 11.2.0
-- Performing Test check_cxx_compiler_flag__Wabi
-- Performing Test check_cxx_compiler_flag__Wabi - Success
-- Performing Test check_cxx_compiler_flag__fabi_version_0
-- Performing Test check_cxx_compiler_flag__fabi_version_0 - Success
-- Performing Test check_cxx_compiler_flag__fabi_compat_version_0
-- Performing Test check_cxx_compiler_flag__fabi_compat_version_0 - Success
-- Performing Test check_cxx_compiler_flag__ffp_contract_fast
-- Performing Test check_cxx_compiler_flag__ffp_contract_fast - Success
-- target changed from "" to "auto"
-- Detected CPU: zen
-- Performing Test check_cxx_compiler_flag__march_znver1
-- Performing Test check_cxx_compiler_flag__march_znver1 - Success
-- Performing Test check_cxx_compiler_flag__msse2
-- Performing Test check_cxx_compiler_flag__msse2 - Success
-- Performing Test check_cxx_compiler_flag__msse3
-- Performing Test check_cxx_compiler_flag__msse3 - Success
-- Looking for C++ include pmmintrin.h
-- Looking for C++ include pmmintrin.h - found
-- Performing Test check_cxx_compiler_flag__mssse3
-- Performing Test check_cxx_compiler_flag__mssse3 - Success
-- Looking for C++ include tmmintrin.h
-- Looking for C++ include tmmintrin.h - found
-- Performing Test check_cxx_compiler_flag__msse4_1
-- Performing Test check_cxx_compiler_flag__msse4_1 - Success
-- Looking for C++ include smmintrin.h
-- Looking for C++ include smmintrin.h - found
-- Performing Test check_cxx_compiler_flag__msse4_2
-- Performing Test check_cxx_compiler_flag__msse4_2 - Success
-- Performing Test check_cxx_compiler_flag__msse4a
-- Performing Test check_cxx_compiler_flag__msse4a - Success
-- Looking for C++ include ammintrin.h
-- Looking for C++ include ammintrin.h - found
-- Performing Test check_cxx_compiler_flag__mavx
-- Performing Test check_cxx_compiler_flag__mavx - Success
-- Looking for C++ include immintrin.h
-- Looking for C++ include immintrin.h - found
-- Performing Test check_cxx_compiler_flag__mfma
-- Performing Test check_cxx_compiler_flag__mfma - Success
-- Performing Test check_cxx_compiler_flag__mbmi2
-- Performing Test check_cxx_compiler_flag__mbmi2 - Success
-- Performing Test check_cxx_compiler_flag__mavx2
-- Performing Test check_cxx_compiler_flag__mavx2 - Success
-- Performing Test check_cxx_compiler_flag__mno_xop
-- Performing Test check_cxx_compiler_flag__mno_xop - Success
-- Performing Test check_cxx_compiler_flag__mno_fma4
-- Performing Test check_cxx_compiler_flag__mno_fma4 - Success
-- Performing Test check_cxx_compiler_flag__mno_avx512f
-- Performing Test check_cxx_compiler_flag__mno_avx512f - Success
-- Performing Test check_cxx_compiler_flag__mno_avx512vl
-- Performing Test check_cxx_compiler_flag__mno_avx512vl - Success
-- Performing Test check_cxx_compiler_flag__mno_avx512pf
-- Performing Test check_cxx_compiler_flag__mno_avx512pf - Success
-- Performing Test check_cxx_compiler_flag__mno_avx512er
-- Performing Test check_cxx_compiler_flag__mno_avx512er - Success
-- Performing Test check_cxx_compiler_flag__mno_avx512cd
-- Performing Test check_cxx_compiler_flag__mno_avx512cd - Success
-- Performing Test check_cxx_compiler_flag__mno_avx512dq
-- Performing Test check_cxx_compiler_flag__mno_avx512dq - Success
-- Performing Test check_cxx_compiler_flag__mno_avx512bw
-- Performing Test check_cxx_compiler_flag__mno_avx512bw - Success
-- Performing Test check_cxx_compiler_flag__mno_avx512ifma
-- Performing Test check_cxx_compiler_flag__mno_avx512ifma - Success
-- Performing Test check_cxx_compiler_flag__mno_avx512vbmi
-- Performing Test check_cxx_compiler_flag__mno_avx512vbmi - Success
-- Found VDT: /cvmfs/sft.cern.ch/lcg/releases/LCG_101_ATLAS_2/vdt/0.4.3/x86_64-centos7-gcc11-opt/include
...

But I guess we'll just have to get used to it. (It will not be printed in WorkDir based builds, only in the configuration of AthenaExternals for now.)