[RISCV] Preserve stack space for outgoing arguments when the function contain variable size objects

E.g.

bar (int x)
{
  char p[x];

  push outgoing variables for foo.
  call foo
}

We need to generate stack adjustment instructions for outgoing arguments by
eliminateCallFramePseudoInstr when the function contains variable size
objects to avoid outgoing variables corrupt the variable size object.

Default hasReservedCallFrame will return !hasFP().
We don't want to generate extra sp adjustment instructions when hasFP()
return true, So We override hasReservedCallFrame as !hasVarSizedObjects().

Differential Revision: https://reviews.llvm.org/D43752

llvm-svn: 327938