powerpc/Makefile: Assemble with -me500 when building for E500

Some of the assembly files use instructions specific to BookE or E500,
which are rejected with the now-default -mcpu=powerpc, so we must pass
-me500 to the assembler just as we pass -me200 for E200.

Fixes: 4bf4f42a2feb ("powerpc/kbuild: Set default generic machine type for 32-bit compile")
Signed-off-by: James Clarke <[e-mail]>
Signed-off-by: Michael Ellerman <[e-mail]>

arch/powerpc/Makefile

···
 cpu-as-$(CONFIG_4xx)		+= -Wa,-m405
 cpu-as-$(CONFIG_ALTIVEC)	+= $(call as-option,-Wa$(comma)-maltivec)
 cpu-as-$(CONFIG_E200)		+= -Wa,-me200
+cpu-as-$(CONFIG_E500)		+= -Wa,-me500
 cpu-as-$(CONFIG_PPC_BOOK3S_64)	+= -Wa,-mpower4
 cpu-as-$(CONFIG_PPC_E500MC)	+= $(call as-option,-Wa$(comma)-me500mc)