build: Pass correct disassembly flags in Clang build

On SVR4-derived platforms, the character `/' is treated as a comment
character, which means that it cannot be used in expressions. The
`--divide' option turns `/' into a normal character. This seems to be
needed with our local build of binutils since we don't yet use the
internal assembler/disassembler of the Clang tooling.

Change-Id: I344fc8670fd5d994f3b63308a513dd367aefc7f9
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/5813
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>

src/arch/x86/Makefile.inc

@@ -30,7 +30,7 @@ subdirs-y += smp
 ifeq ($(CONFIG_COMPILER_GCC),y)
 DISASSEMBLY=-Wa,-acdlns
 else
-DISASSEMBLY=
+DISASSEMBLY=-Wa,--divide
 endif
 
 OPTION_TABLE_H:=