Improve s390x CPU support (#25056)

TODO list, copied from the documentation:

- [x] compiler/platform.nim Add os/cpu properties.
- [x] lib/system.nim Add os/cpu to the documentation for system.hostOS
and system.hostCPU.
- [x] ~~compiler/options.nim Add special os/cpu property checks in
isDefined.~~ seems unnecessary; isn't dont for most CPUs
- [x] compiler/installer.ini Add os/cpu to Project.Platforms field.
- [x] lib/system/platforms.nim Add os/cpu.
- [x] ~~std/private/osseps.nim Add os specializations.~~
- [x] ~~lib/pure/distros.nim Add os, package handler.~~
- [x] ~~tools/niminst/makefile.nimf Add os/cpu compiler/linker flags.~~
already done in https://github.com/nim-lang/Nim/pull/20943
- [x] tools/niminst/buildsh.nimf Add os/cpu compiler/linker flags.

For csource:

- [x] have compiler/platform.nim updated
- [x] have compiler/installer.ini updated
- [x] have tools/niminst/buildsh.nimf updated
- [x] 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:

- [x] check relation to https://github.com/nim-lang/Nim/pull/20943

Possible future work:

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

Co-authored-by: ringabout <43030857+ringabout@users.noreply.github.com>

lib/system.nim

@@ -1069,7 +1069,8 @@ const
     ## Possible values:
     ## `"i386"`, `"alpha"`, `"powerpc"`, `"powerpc64"`, `"powerpc64el"`,
     ## `"sparc"`, `"amd64"`, `"mips"`, `"mipsel"`, `"arm"`, `"arm64"`,
-    ## `"mips64"`, `"mips64el"`, `"riscv32"`, `"riscv64"`, `"loongarch64"`.
+    ## `"mips64"`, `"mips64el"`, `"riscv32"`, `"riscv64"`, `"loongarch64"`,
+    ## `"s390x"`.
 
   seqShallowFlag = low(int)
   strlitFlag = 1 shl (sizeof(int)*8 - 2) # later versions of the codegen \