Makefile: Improve compressed debug info support detection

Commit
  10e68b02c8 ("Makefile: support compressed debug info")
added support for compressed debug sections.

Support is detected by checking
- does the compiler support -gz=zlib
- does the assembler support --compressed-debug-sections=zlib
- does the linker support --compressed-debug-sections=zlib

However, the gcc driver's support for this option is somewhat
convoluted. The driver's builtin specs are set based on the version of
binutils that it was configured with. It reports an error if the
configure-time linker/assembler (i.e., not necessarily the actual
assembler that will be run) do not support the option, but only if the
assembler (or linker) is actually invoked when -gz=zlib is passed.

The cc-option check in scripts/Kconfig.include does not invoke the
assembler, so the gcc driver reports success even if it does not support
the option being passed to the assembler.

Because the as-option check passes the option directly to the assembler
via -Wa,--compressed-debug-sections=zlib, the gcc driver does not see
this option and will never report an error.

Combined with an installed version of binutils that is more recent than
the one the compiler was built with, it is possible for all three tests
to succeed, yet an actual compilation with -gz=zlib to fail.

Moreover, it is unnecessary to explicitly pass
--compressed-debug-sections=zlib to the assembler via -Wa, since the
driver will do that automatically when it supports -gz=zlib.

Convert the as-option to just -gz=zlib, simplifying it as well as
performing a better test of the gcc driver's capabilities.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Makefile

@@ -828,7 +828,7 @@ endif
 
 ifdef CONFIG_DEBUG_INFO_COMPRESSED
 DEBUG_CFLAGS	+= -gz=zlib
-KBUILD_AFLAGS	+= -Wa,--compress-debug-sections=zlib
+KBUILD_AFLAGS	+= -gz=zlib
 KBUILD_LDFLAGS	+= --compress-debug-sections=zlib
 endif
 

lib/Kconfig.debug

@@ -229,7 +229,7 @@ config DEBUG_INFO_COMPRESSED
 	bool "Compressed debugging information"
 	depends on DEBUG_INFO
 	depends on $(cc-option,-gz=zlib)
-	depends on $(as-option,-Wa$(comma)--compress-debug-sections=zlib)
+	depends on $(as-option,-gz=zlib)
 	depends on $(ld-option,--compress-debug-sections=zlib)
 	help
 	  Compress the debug information using zlib.  Requires GCC 5.0+ or Clang