Fix several typos in the NDK build system that caused quite a few bugs:

Fix several typos in the NDK build system that caused quite a few bugs:

- generate thumb code by default (type forced ARM generation)

- make the generation of unoptimized binaries with APP_OPTIM := debug actually work
  (the -O0 -g flags were placed before the -Os -mthumb flags for thumb binaries)

- factor common code from build-executable/shared-library/static-library into build-binary.mk
  and install-binary.mk

- properly add a -L$(SYSROOT)/usr/lib line to make LOCAL_LDLIBS := -lz work as advertized
  (in build-binary.mk)

- add a few missing variables from clear-vars.mk