llvm-native: pass LLVM_TABLEGEN/CLANG_TABLEGEN (not *_TABLEGEN_EXE)

Root cause of the native sub-build failure. A from-scratch LLVM build reads
the cache vars LLVM_TABLEGEN / CLANG_TABLEGEN to skip building a native
tblgen when cross-compiling (llvm/cmake/modules/TableGen.cmake: "Native
TableGen executable. Saves building one when cross-compiling"). The recipe
passed *_TABLEGEN_EXE, which that path does NOT read, so LLVM built its own
native tblgen via the CROSS_TOOLCHAIN_FLAGS_NATIVE sub-build. That native
build ran host gcc against Redox relibc headers (leaked via CMAKE_CXX_FLAGS)
and died on host/redox <stdlib.h> conflicts: __deprecated/__noreturn not a
type, duplicate div_t/ldiv_t. Passing the correct var names makes LLVM use the
redoxer toolchain's host tblgens (LLVM 21.1.2) and skip the native build.

lld21/clang21 build against a PREBUILT llvm and resolve tblgen via
find_program on their own LLVM_TABLEGEN_EXE cache var, so they stay correct.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

local/recipes/dev/llvm-native/recipe.toml

@@ -93,20 +93,30 @@ COOKBOOK_CMAKE_FLAGS+=(
     -DLLVM_ENABLE_THREADS=On
     -DLLVM_INCLUDE_EXAMPLES=Off
     -DLLVM_INCLUDE_TESTS=Off
-    # OFF on purpose: this recipe PROVIDES a host-runnable, version-matched
-    # tblgen via LLVM_TABLEGEN_EXE/CLANG_TABLEGEN_EXE below (the redoxer
-    # toolchain's LLVM 21.1.2). LLVM_OPTIMIZED_TABLEGEN=On would ignore those
-    # and force a redundant native sub-build (CROSS_TOOLCHAIN_FLAGS_NATIVE)
-    # that failed to compile <shared_mutex>. With it off, LLVM uses the
-    # provided tblgen and skips the native build entirely. (llvm21, which does
-    # NOT provide a tblgen, keeps it On and builds its own — different case.)
+    # OFF on purpose: this recipe PROVIDES host-runnable, version-matched
+    # tblgens via LLVM_TABLEGEN/CLANG_TABLEGEN below (the redoxer toolchain's
+    # LLVM 21.1.2). LLVM_OPTIMIZED_TABLEGEN=On would build a second optimized
+    # native tblgen instead of using them. Off => LLVM uses what we provide.
     -DLLVM_OPTIMIZED_TABLEGEN=Off
     -DLLVM_TARGET_ARCH=$ARCH
     -DLLVM_TOOLS_INSTALL_DIR=bin
     -DLLVM_UTILS_INSTALL_DIR=bin
     -DUNIX=1
-    -DLLVM_TABLEGEN_EXE=${COOKBOOK_HOST_SYSROOT}/bin/llvm-tblgen
-    -DCLANG_TABLEGEN_EXE=${COOKBOOK_HOST_SYSROOT}/bin/clang-tblgen
+    # CRITICAL: the cache vars LLVM/Clang read to SKIP building a native tblgen
+    # when cross-compiling are LLVM_TABLEGEN / CLANG_TABLEGEN (see
+    # llvm/cmake/modules/TableGen.cmake: "Native TableGen executable. Saves
+    # building one when cross-compiling"). The *_TABLEGEN_EXE names used
+    # previously are NOT read by a from-scratch LLVM build, so LLVM built its
+    # own native tblgen via the CROSS_TOOLCHAIN_FLAGS_NATIVE sub-build — which
+    # ran the HOST gcc against the Redox relibc headers leaked through
+    # CMAKE_CXX_FLAGS and died on host/redox <stdlib.h> conflicts
+    # (__deprecated/__noreturn undeclared, duplicate div_t). Passing the
+    # correct var names makes LLVM use the provided host tblgens (LLVM 21.1.2)
+    # and skip the native sub-build entirely. (lld21/clang21 build against a
+    # PREBUILT llvm and locate tblgen via find_program on LLVM_TABLEGEN_EXE —
+    # a different cache var owned by those projects — so they are correct.)
+    -DLLVM_TABLEGEN=${COOKBOOK_HOST_SYSROOT}/bin/llvm-tblgen
+    -DCLANG_TABLEGEN=${COOKBOOK_HOST_SYSROOT}/bin/clang-tblgen
 )
 
 COOKBOOK_SOURCE="$COOKBOOK_SOURCE/llvm"