[Arm] Modify default tuning of armv8.1-m.main to use Cortex-M55

[Arm] Modify default tuning of armv8.1-m.main to use Cortex-M55

Previously, compiling with -march=armv8.1-m.main would tune for
Cortex-M7.
However, the Cortex-M7 only supports up to Armv7e-M. The Cortex-M55 is
the earliest CPU that supports Armv8.1-M Mainline so is more appropriate.
This also has the effect of changing the branch cost function used, which
will be necessary to correctly prioritise conditional instructions over branches
in the rest of this patch series.

Regression tested on arm-none-eabi.

gcc/ChangeLog

2020-08-04  Omar Tahir  <omar.tahir@arm.com>

	* config/arm/arm-cpus.in (armv8.1-m.main): Tune for Cortex-M55.