"atexit" isn't defined in libc.a for x86/mips (unlike their arm's counterpart).
Include atexit.S otherwise app using atexit() may fail to link statically.
Also, add crtbegin.c to brand crtbegin_static/dynamic.c with .note.ABI-tag
section of given API level. See $NDK/gen-platforms.sh
Change-Id: I2fdbb92119e3ca2da2c1ced6d77e63136ec35c34