Shrink binary sizes when cross-compiling.

In cross-compiling mode:
* Currently, le32-none-ndk-translate don't need libLLVMWrap.
* Only build llc, le32-none-ndk-translate.
* Remove unnecessary libraries when building libLLVM.so.
* Don't build unittests since our libLLVM only has partial components now.
* Use -Os instead of -O3.
* Use -fdata-sections, -ffunction-sections, -fvisibility-inlines-hidden
  and -Wl,--gc-sections.

After this CL, we can reduce code size about 3MB on library.

Change-Id: I80f02fa5fe57a9cca4a5b50691ad83c719da5c59