Kbuild: Add a option to enable dwarf4 v2

I found that a lot of unresolvable variables when using gdb on the
kernel become resolvable when dwarf4 is enabled. So add a Kconfig flag
to enable it.

It definitely increases the debug information size, but on the other
hand this isn't so bad when debug fusion is used.

v2: Use cc-option
Signed-off-by: Andi Kleen <ak@linux.intel.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>

Makefile

@@ -691,6 +691,9 @@ KBUILD_CFLAGS	+= -g
 endif
 KBUILD_AFLAGS	+= -Wa,-gdwarf-2
 endif
+ifdef CONFIG_DEBUG_INFO_DWARF4
+KBUILD_CFLAGS	+= $(call cc-option, -gdwarf-4,)
+endif
 
 ifdef CONFIG_DEBUG_INFO_REDUCED
 KBUILD_CFLAGS 	+= $(call cc-option, -femit-struct-debug-baseonly) \

lib/Kconfig.debug

@@ -158,6 +158,15 @@ config DEBUG_INFO_SPLIT
 	  to know about the .dwo files and include them.
 	  Incompatible with older versions of ccache.
 
+config DEBUG_INFO_DWARF4
+	bool "Generate dwarf4 debuginfo"
+	depends on DEBUG_INFO
+	help
+	  Generate dwarf4 debug info. This requires recent versions
+	  of gcc and gdb. It makes the debug information larger.
+	  But it significantly improves the success of resolving
+	  variables in gdb on optimized code.
+
 config ENABLE_WARN_DEPRECATED
 	bool "Enable __deprecated logic"
 	default y