Enable toolchain build for a generic target

[mkrack]

• Merge GNU arch files for generic and static serial binaries
• Make arch file sourceable
• Sync compiler flags with toolchain

[alazzaro]

@mkrack This PR is very very clever. I see you did for the ARM system (?)
I had a similar problem when building a container for a cross-platform (effectively a cross-compile).
In the past I tried to make a toolchain version within the container to be portable, but I found several places where we take current system flags to compile. For instance, in FFTW we do the following:

cp2k/tools/toolchain/scripts/stage3/install_fftw.sh

Lines 54 to 56 in 83d825b

	grep '\bavx\b' /proc/cpuinfo 1> /dev/null && FFTW_FLAGS="${FFTW_FLAGS} --enable-avx"
	grep '\bavx2\b' /proc/cpuinfo 1> /dev/null && FFTW_FLAGS="${FFTW_FLAGS} --enable-avx2"
	grep '\bavx512f\b' /proc/cpuinfo 1> /dev/null && FFTW_FLAGS="${FFTW_FLAGS} --enable-avx512"

This is something we should not use, suppose I want to build a container on my laptop (AVX2) to run on a different system (AVX512)... I will try to extend your "generic" flag idea.

[mkrack]

Indeed, a (better) cross-compilation support of the CP2K toolchain is missing. The --generic flag could be extended to a --target=<platform> flag including for instance --target=x86-64-generic as generalization.