native xmake toolchain definition for UE-clang (#805)

This change is meant to provide a smoother experience when working on Linux. After this change, the toolchain setup process is now simply

```bash
$ scripts/ue_build_linux/get_ue_toolchain.sh 
```

and then at config time the toolchain is automatically detected if you downloaded it to the default location or have the `UE_TOOLCHAIN_DIR` environment variable set

```bash
xmake config --mode=debug
```

Compared to the old script-based approach this configures the toolchain more precisely, avoiding leakage into unrelated build processes such as when a package manager decides to build something like Ninja locally etc.

1 files changed, 60 insertions, 0 deletions

diff --git a/toolchains/ue_clang.lua b/toolchains/ue_clang.lua
new file mode 100644
index 000000000..8a1efcc04
--- /dev/null
+++ b/toolchains/ue_clang.lua
@@ -0,0 +1,60 @@
+-- Copyright Epic Games, Inc. All Rights Reserved.
+
+-- Custom toolchain for building with the Unreal Engine Linux clang toolchain.
+--
+-- Usage:
+--   xmake config --toolchain=ue-clang [--sdk=/path/to/ue-toolchain]
+--
+-- If --sdk is not given, the toolchain is resolved in order:
+--   1. $UE_TOOLCHAIN_DIR environment variable
+--   2. ~/.ue-toolchain (default get_ue_toolchain.sh download location)
+
+toolchain("ue-clang")
+    set_kind("standalone")
+
+    on_load(function (toolchain)
+        -- Resolve SDK directory: --sdk flag > $UE_TOOLCHAIN_DIR > ~/.ue-toolchain
+        local sdkdir = toolchain:sdkdir()
+        if not sdkdir or sdkdir == "" then
+            sdkdir = os.getenv("UE_TOOLCHAIN_DIR")
+        end
+        if not sdkdir or sdkdir == "" then
+            local default_path = path.join(os.getenv("HOME"), ".ue-toolchain")
+            if os.isdir(default_path) then
+                sdkdir = default_path
+            end
+        end
+        if not sdkdir or sdkdir == "" then
+            return
+        end
+
+        toolchain:config_set("sdkdir", sdkdir)
+
+        local bindir = path.join(sdkdir, "bin")
+
+        -- Compiler and linker tools
+        toolchain:set("toolset", "cc",      path.join(bindir, "clang"))
+        toolchain:set("toolset", "cxx",     path.join(bindir, "clang++"))
+        toolchain:set("toolset", "ld",      path.join(bindir, "clang++"))
+        toolchain:set("toolset", "sh",      path.join(bindir, "clang++"))
+        toolchain:set("toolset", "ar",      path.join(bindir, "llvm-ar"))
+        toolchain:set("toolset", "strip",   path.join(bindir, "llvm-strip"))
+        toolchain:set("toolset", "objcopy", path.join(bindir, "llvm-objcopy"))
+        toolchain:set("toolset", "as",      path.join(bindir, "clang"))
+        toolchain:set("toolset", "ranlib",  path.join(bindir, "llvm-ranlib"))
+
+        -- Sysroot and stdlib flags for both C and C++
+        local sysroot_flag = "--sysroot=" .. sdkdir
+        toolchain:add("cxflags", sysroot_flag, {force = true})
+        toolchain:add("cxflags", "-stdlib=libc++", {force = true})
+
+        -- Linker flags: sysroot and stdlib selection only.
+        -- Static libc++ overrides are applied at project level (xmake.lua) so they
+        -- don't leak into cmake package builds (e.g. sentry-native/crashpad).
+        toolchain:add("ldflags", sysroot_flag, {force = true})
+        toolchain:add("ldflags", "-stdlib=libc++", {force = true})
+
+        toolchain:add("shflags", sysroot_flag, {force = true})
+        toolchain:add("shflags", "-stdlib=libc++", {force = true})
+    end)
+toolchain_end()