arm64: errata: Add -mpc-relative-literal-loads to build flags

GCC6 (and Linaro's 2015.12 snapshot of GCC5) has a new default that uses
adrp/ldr or adrp/add to address literal pools. When CONFIG_ARM64_ERRATUM_843419
is enabled, modules built with this toolchain fail to load:

  module libahci: unsupported RELA relocation: 275

This patch fixes the problem by passing '-mpc-relative-literal-loads'
to the compiler.

Cc: stable@vger.kernel.org
Fixes: df057cc ("arm64: errata: add module build workaround for erratum #843419")
BugLink: http://bugs.launchpad.net/bugs/1533009
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Suggested-by: Christophe Lyon <christophe.lyon@linaro.org>
Signed-off-by: Dann Frazier <dann.frazier@canonical.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/Makefile

@@ -28,6 +28,7 @@ endif
 
 KBUILD_CFLAGS	+= -mgeneral-regs-only $(lseinstr)
 KBUILD_CFLAGS	+= -fno-asynchronous-unwind-tables
+KBUILD_CFLAGS	+= $(call cc-option, -mpc-relative-literal-loads)
 KBUILD_AFLAGS	+= $(lseinstr)
 
 ifeq ($(CONFIG_CPU_BIG_ENDIAN), y)