Use unified syntax to compile with both llvm and gas.

* Remove useless stab debug info directives .func and .endfunc,
  which are not recognized by clang/llvm integrated assembler.
* Replace require8 with REQUIRE8, preserve8 with PRESERVE8.
* Replace LDRNESB with LDRSBNE, LDREQSB with LDRSBEQ, etc.
* Replace VST1 {*.U8} with VST1.8 ..., VLD1 {*.U8} with VLD1.8,
  and VMOV with VMOV.I8.
* New assembler output code is identical or equivalent to old gas output.
  For example, the 3 references of
      LDR      r7, =0x80808080
  in omxdl/arm_neon/vc/m4p10/src_gcc/armVCM4P10_Average_4x_Align_unsafe_s.S
  produced 3 instances of
      .word   0x80808080
  by llvm integrated assembler but only one by gas.

Change-Id: Ifdcd5bee915a7534198d7f219bceab72045f6a22