[arm] Add -mverbose-cost-dump and de-verbosify cost dumps

[arm] Add -mverbose-cost-dump and de-verbosify cost dumps

This patch adds an -mverbose-cost-dump option, similar to the one in aarch64.
It makes the RTX cost dump print the RTX we're costing in the backend, as well as its cost.
This can be distracting in other cost-related RTL dumps like combine's.

So now we don't dump the backend information by default, but provide the power-user option -mverbose-cost-dump
to enable the old verbose dumping.

This option is for GCC developers debugging the compiler only, so no documentation are added.
It's also trivially simple in functionality so no test is added either.

Bootstrapped and tested on arm-none-linux-gnueabihf.

Committed to trunk.

	* config/arm/arm.opt (mverbose-cost-dump): New option.
	* config/arm/arm.c (arm_rtx_costs): Use it.

From-SVN: r255644