* msp430/Makefile.in (crt0-minrt.o, crtn-minrt.o): New.  Build
from crt0.S with -DMINRT.
(CRT_OBJS): Expand.
(crt_%.o): New rule pattern.  Build multiple objects from crt0.S.
* msp430/crt0.S: Further break out functionality.  Support -DMINRT
that omits all init/fini logic.
* msp430/crtn.S: Likewise.
* msp430/msp430-sim.ld: Wildcard all .crt_* sections, sorted.
* msp430/msp430.ld: Likewise.
* msp430/msp430xl-sim.ld: Likewise.