cleanup: distinguish libc variants

In preparation for proper newlib support add the pieces necessary to
tell who supplies the basic functionality: msp430-libc (mspgcc) or
newlib (msp430-elf) possibly augmented by embtextf.
msp430-elf: work around bug in _disable_interrupts()

Current TI-provided intrinsics fail to implement the standard behavior
for the _disable_interrupts() intrinsic, producing an assembly-time
warning from the toolchain and incorrect behavior relative to other
toolchains.

See: http://e2e.ti.com/support/microcontrollers/msp430/f/166/t/348108.aspx