MIPS: VDSO: Explicitly use -fno-asynchronous-unwind-tables

Not every toolchain has -fno-asynchronous-unwind-tables per default on
MIPS. This patch specifies the necessary option explicitly for VDSO
library build.

This prevents the following build failure:
GENVDSO arch/mips/vdso/vdso-image.c
arch/mips/vdso/genvdso: 'arch/mips/vdso/vdso.so.dbg' contains relocation sections
.../arch/mips/vdso/Makefile:84: recipe for target 'arch/mips/vdso/vdso-image.c' failed
Signed-off-by: Robert Schiele <rschiele@gmail.com>
Signed-off-by: Alexander Sverdlin <alexander.sverdlin@nokia.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: "Maciej W. Rozycki" <macro@imgtec.com>
Cc: Alexander Sverdlin <alexander.sverdlin@nokia.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/15127/Signed-off-by: James Hogan <james.hogan@imgtec.com>

arch/mips/vdso/Makefile

@@ -11,6 +11,7 @@ cflags-vdso := $(ccflags-vdso) \
 	$(filter -W%,$(filter-out -Wa$(comma)%,$(KBUILD_CFLAGS))) \
 	-O2 -g -fPIC -fno-strict-aliasing -fno-common -fno-builtin -G 0 \
 	-DDISABLE_BRANCH_PROFILING \
+	$(call cc-option, -fno-asynchronous-unwind-tables) \
 	$(call cc-option, -fno-stack-protector)
 aflags-vdso := $(ccflags-vdso) \
 	-D__ASSEMBLY__ -Wa,-gdwarf-2