enable PIE for dynamically linked executables

Compile all programs on Android with -fPIE and -pie. This enables
PIE (Position Independent Executables), which helps protect Android
applications from exploitation due to memory management bugs.

Stop using the armelf.x linker script. This script hard codes the
load address of the executable, defeating the position independence
PIE requires.

Note: PIE *static* executables are not supported at this time and
require additional linker changes.

Bug: 5323301
Change-Id: Ieafcc9c4f142495847e163881889d371a59d0878