ARC: make the assembly codes compatible

Make the assembly codes compatible with both GNU
and Metaware toolchain.

* replace ".balign" with ".align"
  ".align" assembler directive is supposed by all
  ARC toolchains and it is implemented in a same
  way across ARC toolchains.
* replace "mov_s __certain_reg" with "mov __certain_reg"
  Even though GCC encodes those mnemonics and even real
  HW executes them according to PRM these are restricted
  ones for mov_s and CCAC rightfully refuses to accept
  such mnemonics. So for compatibility and clarity sake
  we switch to 32-bit mov instruction which allows use
  of all those instructions.
* Add "%%" prefix while accessing registers from inline
  ASM as it is required by MWDT.
* Drop "@" prefix while accessing symbols (defined in C
  code) from ASM code as it is required by MWDT.

Signed-off-by: Wayne Ren <wei.ren@synopsys.com>

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