kbuild: Fix debugging info generation for .S files

Change the debuging info generation flag in KBUILD_AFLAGS from '-gdwarf-2' to
'-Wa,--gdwarf-2'.  This will properly generate the debugging info for .S files
when CONFIG_DEBUG_INFO=y.

It seems current gcc does not pass a '--gdwarf-2' option on to the assembler
when '-gdwarf-2' is on its command line (note the differece in the gcc and as
flags).  This change provides the correct assembler flag to gcc, and so does
not rely on gcc to emit a flag for the assembler.

Signed-off-by: Geoff Levand <geoff@infradead.org> for Huawei, Linaro
Signed-off-by: Michal Marek <mmarek@suse.cz>