Fixes build failures with clang:
clang-7: error: unknown argument: '-fPIC -target'
clang-7: error: no such file or directory: '@<(printf %qn -O2'
clang-7: error: no such file or directory: 'x86_64-apple-darwin'
Introduced by 60c5cf9cea in #112449
The `platform` field is pointless nesting: it's just stuff that happens
to be defined together, and that should be an implementation detail.
This instead makes `linux-kernel` and `gcc` top level fields in platform
configs. They join `rustc` there [all are optional], which was put there
and not in `platform` in anticipation of a change like this.
`linux-kernel.arch` in particular also becomes `linuxArch`, to match the
other `*Arch`es.
The next step after is this to combine the *specific* machines from
`lib.systems.platforms` with `lib.systems.examples`, keeping just the
"multiplatform" ones for defaulting.
When invoking a simple Ada program with `gcc` from `gnats10`, the
following warnings are shown:
```
$ gcc -c conftest.adb
gnat1: warning: command-line option ‘-Wformat=1’ is valid for C/C++/ObjC/ObjC++ but not for Ada
gnat1: warning: command-line option ‘-Wformat-security’ is valid for C/C++/ObjC/ObjC++ but not for Ada
gnat1: warning: ‘-Werror=’ argument ‘-Werror=format-security’ is not valid for Ada
$ echo $?
0
```
This is only spammy when compiling Ada programs inside a Nix derivation,
but certain configure scripts (such as the ./configure script from the
gcc that's built by coreboot's `make crossgcc` command) fail entirely
when getting that warning output.
https://nixos.wiki/wiki/Coreboot currently suggests manually running
> NIX_HARDENING_ENABLE="${NIX_HARDENING_ENABLE/ format/}" make crossgcc
… but actually teaching the nixpkgs-provided cc wrapper that `format`
isn't supported as a hardening flag seems to be the more canonical way
to do this in nixpgks.
After this, Ada programs still compile:
```
$ gcc -c conftest.adb
$ echo $?
0
```
And the compiler output is empty.
We need to set FC so that CMake and other tools can find the fortran
compiler. Also we need to limit the hardening flags since fortify and
format don’t work with fortran.
Fixes#88449
I hate the thing too even though I made it, and rather just get rid of
it. But we can't do that yet. In the meantime, this brings us more
inline with autoconf and will make it slightly easier for me to write a
pkg-config wrapper, which we need.
Regression introduced in PR #8119180729b6787. The file does not exist
somewhere during bootstrap of pkgsStatic.busybox which is used in nix
(by default).
I tested the builds.
