[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
diff --git a/llvm/test/CodeGen/MSP430/fp.ll b/llvm/test/CodeGen/MSP430/fp.ll
index e7d7c51..bf60370 100644
--- a/llvm/test/CodeGen/MSP430/fp.ll
+++ b/llvm/test/CodeGen/MSP430/fp.ll
@@ -27,3 +27,5 @@
   call void asm sideeffect "nop", "r"(i8 0)
   ret void
 }
+
+attributes #0 = { noinline nounwind optnone "interrupt"="2" }