pru: Implement TARGET_INSN_COST

pru: Implement TARGET_INSN_COST

This patch slightly improves the embench-iot benchmark score for
PRU code size.  There is also small improvement in a few real-world
firmware programs.

  Embench-iot size
  ------------------------------------------
  Benchmark          before   after    delta
  ---------           ----    ----     -----
  aha-mont64          4.15    4.15         0
  crc32               6.04    6.04         0
  cubic              21.64   21.62     -0.02
  edn                 6.37    6.37         0
  huffbench          18.63   18.55     -0.08
  matmult-int         5.44    5.44         0
  md5sum             25.56   25.43     -0.13
  minver             12.82   12.76     -0.06
  nbody              15.09   14.97     -0.12
  nettle-aes          4.75    4.75         0
  nettle-sha256       4.67    4.67         0
  nsichneu            3.77    3.77         0
  picojpeg            4.11    4.11         0
  primecount          7.90    7.90         0
  qrduino             7.18    7.16     -0.02
  sglib-combined     13.63   13.59     -0.04
  slre                5.19    5.19         0
  st                 14.23   14.12     -0.11
  statemate           2.34    2.34         0
  tarfind            36.85   36.64     -0.21
  ud                 10.51   10.46     -0.05
  wikisort            7.44    7.41     -0.03
  ---------          -----   -----
  Geometric mean      8.42    8.40     -0.02
  Geometric SD        2.00    2.00         0
  Geometric range    12.68   12.62     -0.06

gcc/ChangeLog:

	* config/pru/pru.cc (pru_insn_cost): New function.
	(TARGET_INSN_COST): Define for PRU.

Signed-off-by: Dimitar Dimitrov <dimitar@dinux.eu>