Fixes for "Shrink binary sizes when cross-compiling."

.. LLVM_CROSS_COMPILING isn't the appropriate test before enabling
-fdata-sections, -ffunction-sections, -fvisibility-inlines-hidden or
-Wl,--gc-sections. The first three should be enabled if the compiler
supports those flags and the last one if the linker supports
--gc-sections.

Setting these flags when LLVM_CROSS_COMPILING caused cross-builds to
fail with my Darwin-targeting toolchain.

Change-Id: I1e894449ef31dabe3810d9a3f2fabbf6f2e52230