Kconfig: move LTO options to Build Setup menu

LTO is optimization option so it's more appropriate to place it
in the same place as other optimization options

Kconfig

@@ -2432,6 +2432,44 @@ config DEBUG_OPTLEVEL
 		This string represents the custom optimization level that will be
 		used if DEBUG_CUSTOMOPT.
 
+choice
+	prompt "Link Time Optimization (LTO)"
+	default LTO_NONE
+	---help---
+		This option enables Link Time Optimization (LTO), which allows the
+		compiler to optimize binaries globally.
+
+		If unsure, select LTO_NONE. Note that LTO is very resource-intensive
+		so it's disabled by default.
+
+config LTO_NONE
+	bool "None"
+	---help---
+		Build the kernel normally, without Link Time Optimization (LTO).
+
+config LTO_FULL
+	bool "GNU Full LTO (EXPERIMENTAL)"
+	depends on ARCH_TOOLCHAIN_GNU
+	---help---
+		Link time optimization is implemented as a GCC front end for a bytecode
+		bytecode representation of GIMPLE that is emitted in special sections
+		of .o files. Currently, LTO support is enabled in most ELF-based systems,
+		as well as darwin, cygwin and mingw systems.
+
+config LTO_THIN
+	bool "Clang ThinLTO (EXPERIMENTAL)"
+	depends on ARCH_TOOLCHAIN_CLANG
+	---help---
+		This option enables Clang's ThinLTO, which allows for parallel
+		optimization and faster incremental compiles compared to the
+		CONFIG_LTO_FULL option. More information can be found
+		from Clang's documentation:
+
+		https://clang.llvm.org/docs/ThinLTO.html
+
+		If unsure, say Y.
+endchoice
+
 config DEBUG_OPT_UNUSED_SECTIONS
 	bool "Optimization to eliminate the unused input sections"
 	default y