llvm-tools: build host llvm-objcopy/strip/profdata under ZigToolchain

Add an `llvm-tools` target artifact that downloads llvm-project source
matching the version of clang shipped by the active zig install,
builds llvm-objcopy, llvm-strip and llvm-profdata into
PKG_ROOT_PATH/llvm-tools/bin, and exposes them through the same
path/binary/isInstalled static surface as the other artifacts.

A new LlvmToolsCheck doctor item runs when the active toolchain is
ZigToolchain and reports whether the three tools are built, with a
fix that installs the package and runs the build.

PackageBuilder now picks the right tool when the active toolchain is
ZigToolchain:
- extractDebugInfo() honours OBJCOPY from the environment, then falls
  back to llvm-tools' llvm-objcopy under Zig and plain objcopy
  otherwise.
- stripBinary() uses llvm-strip under Zig and plain strip otherwise.

System strip/objcopy refuse zig-produced archives and bitcode
sections, so without this the strip stage breaks LTO builds. Other
toolchains keep using the system binaries.

ApplicationContext::tryGet() wraps the container's get() in a
try/catch and returns null on failure, so PackageBuilder can ask
"which toolchain is active right now" without PHP-DI throwing on
autowirable-but-unconstructable classes.

Depends on v3c/artifact-static-helpers (uses zig::isInstalled()
and zig::binary()).

config/pkg/target/llvm-tools.yml

@@ -0,0 +1,6 @@
+llvm-tools:
+  type: target
+  artifact:
+    binary: custom
+  depends:
+    - zig