aarch64: Define TARGET_INSN_COST

This patch adds a bare-bones TARGET_INSN_COST.  See the comment
in the patch for the rationale.

Just to get a flavour for how much difference it makes, I tried
compiling the testsuite with -Os -fno-schedule-insns{,2} and
seeing what effect the patch had on the number of instructions.
Very few tests changed, but all the changes were positive:

  Tests   Good    Bad   Delta    Best   Worst  Median
  =====   ====    ===   =====    ====   =====  ======
     19     19      0    -177     -52      -1      -4

The change for -O2 was even smaller, but more mixed:

  Tests   Good    Bad   Delta    Best   Worst  Median
  =====   ====    ===   =====    ====   =====  ======
      6      3      3      -8      -9       6      -2

There were no obvious effects on SPEC CPU2017.

The patch is needed to avoid a regression with a later change.

gcc/
	* config/aarch64/aarch64.cc (aarch64_insn_cost): New function.
	(TARGET_INSN_COST): Define.