all: LLVMLinux: Change DWARF flag to support gcc and clang

Both gcc (well, actually gnu as) and clang support the "-Wa,-gdwarf-2" option
(though clang does not support "-Wa,--gdwarf-2"). Since these flags are equivalent
in meaning, this patch uses the one which is better supported across compilers.
Signed-off-by: Behan Webster <behanw@converseincode.com>

Makefile

@@ -671,7 +671,7 @@ endif
 
 ifdef CONFIG_DEBUG_INFO
 KBUILD_CFLAGS	+= -g
-KBUILD_AFLAGS	+= -Wa,--gdwarf-2
+KBUILD_AFLAGS	+= -Wa,-gdwarf-2
 endif
 
 ifdef CONFIG_DEBUG_INFO_REDUCED