MIPS: Clean up arch/mips/boot/compressed/ld.script

MIPS: Clean up arch/mips/boot/compressed/ld.script

- Remove unused symbols: _fdata, _text; only _edata and _end are needed by
  head.S
- Remove unused sections: .sbss, .stab, .gptab.sdata, .gptab.sbss
- Change the alignment to 16 bytes to ensure it is greater than any
  fundamental type of a MIPS compiler.
- Clean up comments

Signed-off-by: Wu Zhangjin <wuzhangjin@gmail.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/1381/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>