freebsd.libcxxrt: fixup in preparation for inclusion in stdenv

- add option to build without libcxx
- don't build with libcxx
- distribute headers

pkgs/os-specific/bsd/freebsd/patches/14.0/libcxxrt-headers.patch

@@ -0,0 +1,11 @@
+--- freebsd/lib/libcxxrt/Makefile	2024-05-30 14:27:42.328086005 -0700
++++ freebsd/lib/libcxxrt/Makefile.mod	2024-05-30 14:27:48.048014581 -0700
+@@ -19,6 +19,8 @@
+ SRCS+=		terminate.cc
+ SRCS+=		typeinfo.cc
+ 
++INCS+=cxxabi.h unwind.h unwind-arm.h unwind-itanium.h
++
+ WARNS?=		0
+ CFLAGS+=	-isystem ${SRCDIR} -nostdinc++
+ CXXSTD?=	c++14

pkgs/os-specific/bsd/freebsd/pkgs/libcxxrt.nix

@@ -1,4 +1,4 @@
-{ mkDerivation, ... }:
+{ mkDerivation }:
 # this package is quite different from stock libcxxrt.
 # as of FreeBSD 14.0, it is vendored from APPROXIMATELY libcxxrt
 # 5d8a15823a103bbc27f1bfdcf2b5aa008fab57dd, though the vendoring mechanism is
@@ -8,4 +8,11 @@ mkDerivation {
   pname = "libcxxrt";
   path = "lib/libcxxrt";
   extraPaths = [ "contrib/libcxxrt" ];
+  outputs = [
+    "out"
+    "dev"
+    "debug"
+  ];
+  noLibcxx = true;
+  libName = "cxxrt";
 }

pkgs/os-specific/bsd/freebsd/pkgs/mkDerivation.nix

@@ -3,6 +3,8 @@
   stdenv,
   stdenvNoCC,
   stdenvNoLibs,
+  overrideCC,
+  buildPackages,
   versionData,
   writeText,
   patches,
@@ -27,6 +29,8 @@ lib.makeOverridable (
         stdenvNoCC
       else if attrs.noLibc or false then
         stdenvNoLibs
+      else if attrs.noLibcxx or false then
+        overrideCC stdenv buildPackages.llvmPackages.clangNoLibcxx
       else
         stdenv;
   in