clang: don't set -march for overridden target

If -target is explicitly passed to clang, we shouldn't pass our -march
value for the default target, because it probably won't exist for the
target being used.  Up until now, clang has been lenient with this,
but it's a hard error with clang 17, so since gcc.arch is always set
on aarch64, fixing this is a hard requirement for upgrading our
default clang to 17.

Before (with clang 17 on aarch64-linux):

	$ clang -target bpf -c -o /dev/null test.bpf.c
	clang: warning: ignoring '-fstack-protector-strong' option as it is not currently supported for target 'bpf' [-Woption-ignored]
	clang: error: unsupported option '-march=' for target 'bpf'
	clang: warning: argument unused during compilation: '--gcc-toolchain=/nix/store/cngak08nb1yk44gnjipv5rg1ahh1xkax-gcc-13.2.0' [-Wunused-command-line-argument]

After:

	$ clang -target bpf -c -o /dev/null test.bpf.c
	clang: warning: ignoring '-fstack-protector-strong' option as it is not currently supported for target 'bpf' [-Woption-ignored]
	clang: warning: argument unused during compilation: '--gcc-toolchain=/nix/store/cngak08nb1yk44gnjipv5rg1ahh1xkax-gcc-13.2.0' [-Wunused-command-line-argument]

pkgs/build-support/cc-wrapper/add-clang-cc-cflags-before.sh

@@ -7,5 +7,5 @@ for p in "${params[@]}"; do
 done
 
 if $needsTarget; then
-    extraBefore+=(-target @defaultTarget@)
+    extraBefore+=(-target @defaultTarget@ @march@)
 fi

pkgs/build-support/cc-wrapper/default.nix

@@ -595,8 +595,11 @@ stdenv.mkDerivation {
     # Always add -march based on cpu in triple. Sometimes there is a
     # discrepency (x86_64 vs. x86-64), so we provide an "arch" arg in
     # that case.
+    #
+    # For clang, this is handled in add-clang-cc-cflags-before.sh
+
     # TODO: aarch64-darwin has mcpu incompatible with gcc
-    + optionalString ((targetPlatform ? gcc.arch) && (isClang || !(stdenv.isDarwin && stdenv.isAarch64)) &&
+    + optionalString ((targetPlatform ? gcc.arch) && !isClang && !(stdenv.isDarwin && stdenv.isAarch64) &&
                       isGccArchSupported targetPlatform.gcc.arch) ''
       echo "-march=${targetPlatform.gcc.arch}" >> $out/nix-support/cc-cflags-before
     ''
@@ -685,6 +688,10 @@ stdenv.mkDerivation {
     ## Needs to go after ^ because the for loop eats \n and makes this file an invalid script
     ##
     + optionalString isClang ''
+      # Escape twice: once for this script, once for the one it gets substituted into.
+      export march=${lib.escapeShellArg
+        (lib.optionalString (targetPlatform ? gcc.arch)
+          (lib.escapeShellArg "-march=${targetPlatform.gcc.arch}"))}
       export defaultTarget=${targetPlatform.config}
       substituteAll ${./add-clang-cc-cflags-before.sh} $out/nix-support/add-local-cc-cflags-before.sh
     ''