Add an option --enable-shrink-binary-size to configure.

This option could reduce the LLVM tools by "-fdata-sections, -ffunction-sections"
and "--gc-sections". Note that with --enable-shrink-binary-size, your LLVM tools
might not be able to load plugins (ex: LLVMPolly).

The default is off to make NDK llvm tools support plugin.

This commit mirrors two commits in the Android LLVM git repo, from release_33
branch:

1: Ray Donnelly <mingw.android@gmail.com>
6230037 Fixes for "Shrink binary sizes when cross-compiling."

2: Lai Wei-Chih <Robert.Lai@mediatek.com>
0190585 Add an option --enable-shrink-binary-size to configure.