Rollup merge of #93756 - tmandry:llvm-build-config, r=Mark-Simulacrum

Support custom options for LLVM build

The LLVM build has a lot of options that rustbuild doesn't need to know about. We should allow the user to customize the LLVM build directly.

Here are some [example customizations][recipe] we'd like to do.

[recipe]: 90105e5e4e/recipes/contrib/clang_toolchain.py (579)

config.toml.example

@@ -157,6 +157,9 @@ changelog-seen = 2
 # Whether to build the clang compiler.
 #clang = false
 
+# Custom CMake defines to set when building LLVM.
+#build-config = {}
+
 # =============================================================================
 # General build configuration options
 # =============================================================================