Auto merge of #43099 - japaric:msp430, r=alexcrichton

add a built-in MSP430 target

the MSP430 backend has been enabled for a while but no target was added to rustc
to encourage out of tree experimentation.

We believe the out of tree (custom) target has been iterated long enough and is
stable enough for inclusion in the compiler. Kudos to @pftbest and @awygle for
fixing several LLVM / codegen bugs this target had!

The target name chosen is a slight variation of the triple gcc uses, which is
simply `msp430-elf`. We picked `msp430-none-elf` to leave room for custom
targets that target some embedded OS running on MSP430 devices. (cf. the
custom `thumbv7m-tockos-eabi` target TockOS uses vs the built-in
`thumbv7m-none-eabi`).

There's one expected change in the specification of the proposed target: the
`asm_args` and `no_integrated_as` fields will change to their default values.
Once the LLVM backend gains the ability to directly produce MSP430 object files
we can stop depending on `msp430-elf-gcc` for producing object files; when that
occurs the `asm` related fields will change. This change won't break existing
user code.

r? @alexcrichton
cc @brson

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