[MSP430] Emit a separate section for every interrupt vector

This is LLVM part of D56663

Linker scripts shipped by TI require to have every
interrupt vector in a separate section with a specific name:

 SECTIONS
 {
   __interrupt_vector_XX   : { KEEP (*(__interrupt_vector_XX )) } > VECTXX
   ...
 }

Follow the requirement emit the section for every vector
which contain address of interrupt handler:

  .section  __interrupt_vector_XX,"ax",@progbits
  .word %isr%

Patch by Kristina Bessonova!

Differential Revision: https://reviews.llvm.org/D56664

llvm-svn: 351345