Improve s390x CPU support

[neuschaefer]

TODO list, copied from the documentation:

• compiler/platform.nim Add os/cpu properties.
• lib/system.nim Add os/cpu to the documentation for system.hostOS and system.hostCPU.
• compiler/options.nim Add special os/cpu property checks in isDefined. seems unnecessary; isn't dont for most CPUs
• compiler/installer.ini Add os/cpu to Project.Platforms field.
• lib/system/platforms.nim Add os/cpu.
• std/private/osseps.nim Add os specializations.
• lib/pure/distros.nim Add os, package handler.
• tools/niminst/makefile.nimf Add os/cpu compiler/linker flags. already done in Support IBM Z architecture #20943
• tools/niminst/buildsh.nimf Add os/cpu compiler/linker flags.

For csource:

• have compiler/platform.nim updated
• have compiler/installer.ini updated
• have tools/niminst/buildsh.nimf updated
• have tools/niminst/makefile.nimf updated
• be backported to the Nim version used by the csources
• the new csources must be pushed
• the new csources revision must be updated in config/build_config.txt

Additionally:

• check relation to Support IBM Z architecture #20943

Possible future work:

• Porting Nim to s390x-specific operating systems, notably z/OS

[neuschaefer]

Pushing the new csources would make it possible to build Nim on s390x without any tricks and manual edits, but I don't know how to re-generate the csources from the Nim repo.