aarch64:vfp: fix vfp ctx changed in kernel

issue: #16

--------
1 Exception doesn't save VFP registers
2 kernel generate code shouldn't use VFP registers,
  need close -neon and -fp-armv8
3 taskctx need save/restore user application VFP ctx,
  need access VFP registers

After rustc update, it would complain when 2&3 happend,
on this commit :65a4f5896317115cf1edc74800f0da5d92923243,
opened neon when fp_smid enable,it broken 2, so it create
bug; after return user from kernel trap, usr vfp registers
ctx are changed;

This commit close neon when complie kernel, taskctx need access
VFP registers through build.rs to fix

Signed-off-by: guoweikang <guoweikang.kernel@gmail.com>

Makefile

@@ -122,11 +122,7 @@ ifeq ($(ARCH), x86_64)
 else ifeq ($(ARCH), riscv64)
   TARGET := riscv64gc-unknown-none-elf
 else ifeq ($(ARCH), aarch64)
-  ifeq ($(findstring fp_simd,$(FEATURES)),)
-    TARGET := aarch64-unknown-none-softfloat
-  else
-    TARGET := aarch64-unknown-none
-  endif
+   TARGET := aarch64-unknown-none-softfloat
 endif
 
 export AX_ARCH=$(ARCH)