mips: vdso: fix DWARF2 warning

When building mips tinyconifg the following warning show up

make --silent --keep-going --jobs=8 O=/home/anders/src/kernel/next/out/builddir ARCH=mips CROSS_COMPILE=mips-linux-gnu- HOSTCC=clang CC=clang
/srv/src/kernel/next/arch/mips/vdso/elf.S:14:1: warning: DWARF2 only supports one section per compilation unit
.pushsection .note.Linux, "a",@note ; .balign 4 ; .long 2f - 1f ; .long 4484f - 3f ; .long 0 ; 1:.asciz "Linux" ; 2:.balign 4 ; 3:
^
/srv/src/kernel/next/arch/mips/vdso/elf.S:34:2: warning: DWARF2 only supports one section per compilation unit
 .section .mips_abiflags, "a"
 ^

Rework so the mips vdso Makefile adds flag '-no-integrated-as' unless
LLVM_IAS is defined.

Link: ClangBuiltLinux#1256
Cc: stable@vger.kernel.org # v4.19+
Suggested-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>

arch/mips/vdso/Makefile

@@ -26,6 +26,10 @@ ifdef CONFIG_CC_IS_CLANG
 ccflags-vdso += $(filter --target=%,$(KBUILD_CFLAGS))
 endif
 
+ifneq ($(LLVM_IAS),1)
+ccflags-vdso += -no-integrated-as
+endif
+
 #
 # The -fno-jump-tables flag only prevents the compiler from generating
 # jump tables but does not prevent the compiler from emitting absolute