Fix a bug that MIPS thunks can overwrite other section contents.

Peter Smith found while trying to support thunk creation for ARM that
LLD sometimes creates broken thunks for MIPS. The cause of the bug is
that we assign file offsets to input sections too early. We need to
create all sections and then assign section offsets because appending
thunks changes file offsets for all following sections.

This patch separates the pass to assign file offsets from thunk
creation pass. This effectively reverts r265673.

Differential Revision: http://reviews.llvm.org/D21598

llvm-svn: 273532