Add an explicit definition of __ANDROID__=1

Once the aarch64 toolchain is refreshed this will be removed.
Also removed some CFLAG duplications.

Change-Id: I42fd86931263af82042ea696196fb25e948fe9cb

Conflicts:
	core/combo/TARGET_linux-aarch64.mk
enable PIE for dynamically linked Linux host executables

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

Note: PIE *static* executables are not supported at this time. darwin
and windows are also not supported.

Change-Id: I67c1be297711ea23064c349f97ebd02eef3eff09