gcc: extend crtn workaround to alpha target

Without the change `alpha-unknown-linux-gnu` target is failing to build
`gcc`:

```
$ NIXPKGS_ALLOW_UNSUPPORTED_SYSTEM=1 nix build -f ./. re2c --arg crossSystem '{ config = "alpha-unknown-linux-gnu"; }'
...
make[1]: *** No rule to make target '../../../gcc-12.3.0/libgcc/config/alpha/crti.S', needed by 'crti.o'. Stop.
...
cc1: error: fp software completion requires '-mtrap-precision=i' [-Werror]
```

After the change it is able to produce working binaries:

```
$ NIXPKGS_ALLOW_UNSUPPORTED_SYSTEM=1 nix build -f ./. re2c --arg crossSystem '{ config = "alpha-unknown-linux-gnu"; }'
...
$ qemu-alpha ./result/bin/re2c --version
re2c 3.1

$ file result/bin/re2c
result/bin/re2c: ELF 64-bit LSB executable, Alpha (unofficial), version 1 (SYSV), dynamically linked, interpreter ...-glibc-alpha-unknown-linux-gnu-2.38-27/lib/ld-linux.so.2, for GNU/Linux 3.10.0, not stripped
```

pkgs/development/compilers/gcc/common/configure-flags.nix

···
     ++ lib.optionals (targetPlatform.isMips && targetPlatform.parsed.abi.name == "gnu" && lib.versions.major version == "12") [
       "--disable-libsanitizer"
     ]
+    ++ lib.optionals targetPlatform.isAlpha [
+      # Workaround build failures like:
+      #   cc1: error: fp software completion requires '-mtrap-precision=i' [-Werror]
+      "--disable-werror"
+    ]
   ;
 
 in configureFlags

pkgs/development/compilers/gcc/common/libgcc-buildstuff.nix

···
   # https://www.openwall.com/lists/musl/2022/11/09/3
   #
   # 'parsed.cpu.family' won't be correct for every platform.
-+ lib.optionalString (stdenv.targetPlatform.isLoongArch64 || stdenv.targetPlatform.isS390) ''
++ lib.optionalString (stdenv.targetPlatform.isLoongArch64 || stdenv.targetPlatform.isS390 || stdenv.targetPlatform.isAlpha) ''
   touch libgcc/config/${stdenv.targetPlatform.parsed.cpu.family}/crt{i,n}.S
 ''