ELF: Add -O0 (produce output as fast as possible) mode.

This patch redefines the default optimization level as 1 and adds
new level 0. In the command line, it is -O0. The flag disables
costly but optional features so that the linker produces semantically
correct but larger output quickly. Currently it only disables
section merging.

This flag is not intended to be used for final production linking.
It is intended to be used in compile-link-test cycle.

Time to link clang with debug info is about 2x faster with the flag.

  Head:
  13.24 seconds
  Output size: 1227189664 bytes

  With this patch:
  7.41 seconds
  Output size: 2490281784 bytes

Differential Revision: http://reviews.llvm.org/D19705

llvm-svn: 268056