llvmPackages_15.compiler-rt: gate the libxcrypt dep on plat.libc == "glibc"

This restores this check to what it originally was in #196909 (see:
https://github.com/NixOS/nixpkgs/pull/196909#discussion_r1000536298) and
lets `compiler-rt` eval successfully when trying to compile the
`llvmPackages_15` set for mingw targets (i.e. a platform that *is* GNU
but does *not* use glibc).

---

It's not clear to me what the `haveLibc` check is doing here (platforms
that seem to use glibc like `x86_64-linux` and have
`plat.libc == "glibc"` have `haveLibc = false` because `stdenv.cc.libc`
is `null`).

pkgs/development/compilers/llvm/15/compiler-rt/default.nix

@@ -39,7 +39,7 @@ stdenv.mkDerivation {
     "-DCOMPILER_RT_DEFAULT_TARGET_ONLY=ON"
     "-DCMAKE_C_COMPILER_TARGET=${stdenv.hostPlatform.config}"
     "-DCMAKE_ASM_COMPILER_TARGET=${stdenv.hostPlatform.config}"
-  ] ++ lib.optionals (haveLibc && isGnu) [
+  ] ++ lib.optionals (haveLibc && stdenv.hostPlatform.libc == "glibc") [
     "-DSANITIZER_COMMON_CFLAGS=-I${libxcrypt}/include"
   ] ++ lib.optionals (useLLVM || bareMetal || isMusl) [
     "-DCOMPILER_RT_BUILD_SANITIZERS=OFF"