kbuild: pass -g to assembler under CONFIG_DEBUG_INFO

The assembler for a while now supports -gdwarf to generate source line info
just like the C compiler does.  Source-level assembly debugging sounds like an
oxymoron, but it is handy to be able to see the right source file and read its
comments rather than just the disassembly.  This patch enables -gdwarf for
assembly files when CONFIG_DEBUG_INFO=y and the assembler supports the option.

Signed-off-by: Roland McGrath <roland@redhat.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: <linux-arch@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>