MSP430: Use enums to handle -mcpu= values

MSP430: Use enums to handle -mcpu= values

The -mcpu= option accepts only a handful of string values.
Using enums instead of strings to handle the accepted values removes the
need to have specific processing of the strings in the backend, and
simplifies any comparisons which need to be performed on the value.

It also allows the default value to have semantic equivalence to a user
set value, whilst retaining the ability to differentiate between them.
Practically, this allows a user set -mcpu= value to override the the ISA set by
-mmcu, whilst the default -mcpu= value can still have an explicit meaning.

gcc/ChangeLog:

	* common/config/msp430/msp430-common.c (msp430_handle_option): Remove
	OPT_mcpu_ handling.
	Set target_cpu value to new enum values when parsing certain -mmcu=
	values.
	* config/msp430/msp430-opts.h (enum msp430_cpu_types): New.
	* config/msp430/msp430.c (msp430_option_override): Handle new
	target_cpu enum values.
	Set target_cpu using extracted value for given MCU when -mcpu=
	option is not passed by the user.
	* config/msp430/msp430.opt: Handle -mcpu= values using enums.

gcc/testsuite/ChangeLog:

	* gcc.target/msp430/mcpu-is-430.c: New test.
	* gcc.target/msp430/mcpu-is-430x.c: New test.
	* gcc.target/msp430/mcpu-is-430xv2.c: New test.