build: support building with Link Time Optimizations

Add plumbing for building U-Boot with Link Time Optimizations.

When building with LTO, $(PLATFORM_LIBS) has to be in --whole-archive /
--no-whole-archive group, otherwise some functions declared in assembly
may not be resolved and linking may fail.

Note: clang may throw away linker list symbols it thinks are unused when
compiling with LTO. To force these symbols to be included, we refer to
them via the __ADDRESSABLE macro in a C file generated from compiled
built-in.o files before linking.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Simon Glass <sjg@chromium.org>

Kconfig

@@ -85,6 +85,30 @@ config SPL_OPTIMIZE_INLINING
 	  do what it thinks is best, which is desirable in some cases for size
 	  reasons.
 
+config ARCH_SUPPORTS_LTO
+	bool
+
+config LTO
+	bool "Enable Link Time Optimizations"
+	depends on ARCH_SUPPORTS_LTO
+	default n
+	help
+	  This option enables Link Time Optimization (LTO), a mechanism which
+	  allows the compiler to optimize between different compilation units.
+
+	  This can optimize away dead code paths, resulting in smaller binary
+	  size (if CC_OPTIMIZE_FOR_SIZE is enabled).
+
+	  This option is not available for every architecture and may
+	  introduce bugs.
+
+	  Currently, when compiling with GCC, due to a weird bug regarding
+	  jobserver, the final linking will not respect make's --jobs argument.
+	  Instead all available processors will be used (as reported by the
+	  nproc command).
+
+	  If unsure, say n.
+
 config TPL_OPTIMIZE_INLINING
 	bool "Allow compiler to uninline functions marked 'inline' in TPL"
 	depends on TPL