ANDROID: kbuild: add support for Clang LTO

This change adds the configuration option CONFIG_LTO_CLANG, and
build system support for Clang's Link Time Optimization (LTO). In
preparation for LTO support with other compilers, potentially common
parts of the changes are gated behind CONFIG_LTO instead.

With -flto, instead of object files, Clang produces LLVM bitcode,
which is compiled into a native object at link time, allowing the
final binary to be optimized globally. For more details, see:

  https://llvm.org/docs/LinkTimeOptimization.html

While the kernel normally uses GNU ld for linking, LLVM supports LTO
only with LLD or GNU gold linkers. This change assumes LLD is used.

Bug: 145210207
Change-Id: If1164ff33d073358ee7d4bba84cbb06c349c4a88
Signed-off-by: Sami Tolvanen <[email protected]>

arch/Kconfig

@@ -554,6 +554,53 @@ config SHADOW_CALL_STACK_VMAP
 	  provides better stack exhaustion protection, but increases per-thread
 	  memory consumption as a full page is allocated for each shadow stack.
 
+config LTO
+	bool
+
+config ARCH_SUPPORTS_LTO_CLANG
+	bool
+	help
+	  An architecture should select this option if it supports:
+	  - compiling with Clang,
+	  - compiling inline assembly with Clang's integrated assembler,
+	  - and linking with LLD.
+
+config ARCH_SUPPORTS_THINLTO
+	bool
+	help
+	  An architecture should select this if it supports Clang ThinLTO.
+
+config THINLTO
+	bool "Use Clang's ThinLTO (EXPERIMENTAL)"
+	depends on LTO_CLANG && ARCH_SUPPORTS_THINLTO
+	default y
+	help
+	  Use ThinLTO to speed up Link Time Optimization.
+
+choice
+	prompt "Link-Time Optimization (LTO) (EXPERIMENTAL)"
+	default LTO_NONE
+	help
+	  This option turns on Link-Time Optimization (LTO).
+
+config LTO_NONE
+	bool "None"
+
+config LTO_CLANG
+	bool "Use Clang's Link Time Optimization (LTO) (EXPERIMENTAL)"
+	depends on ARCH_SUPPORTS_LTO_CLANG
+	depends on !KASAN
+	depends on !FTRACE_MCOUNT_RECORD
+	depends on CC_IS_CLANG && CLANG_VERSION >= 100000 && LD_IS_LLD
+	select LTO
+	help
+          This option enables Clang's Link Time Optimization (LTO), which allows
+          the compiler to optimize the kernel globally at link time. If you
+          enable this option, the compiler generates LLVM IR instead of object
+          files, and the actual compilation from IR occurs at the LTO link step,
+          which may take several minutes.
+
+endchoice
 
 config HAVE_ARCH_WITHIN_STACK_FRAMES
 	bool