[MSP430][Clang] Remove support for -mmcu=msp430

The -mmcu= option accepts a generic MCU named "msp430", which sets the
CPU to msp430 and disables hardware multiply support.

The current purpose of accepting this value is to allow -mmcu= to be
used as an alias for -mcpu=, however there are some downsides to doing
this. -mmcu= provides additional features that will interfere
with the expected behavior if the user tries to to use it as an alias
for -mcpu=.

-mmcu=msp430 will conflict with -mhwmult=, since the "msp430" MCU is
defined to have no hardware multiply support, so the user will not be
able to set an explicit hardware multiply version.

-mmcu=msp430 will put "-Tmsp430.ld" on the linker command line, however
TI's support files do not provide a linker script with this name and so
the user would have to explicitly create it.

Differential Revision: https://reviews.llvm.org/D108299