Remove outdated advice from the readme

We no longer use the GCCVERSION variable - we let the NDK pick whichever
compiler version is the default for the chosen architecture.
Remove unused includes of sys/timeb.h

This header is included by crt_util_safe_x.h whenever necessary (it's
all hidden in the SWelsTime struct) - code which does not explicitly
use things from sys/timeb.h shouldn't include it.
Manually define the SWelsTime struct on unix, don't rely on struct timeb

We don't use the ftime function on unix for filling SWelsTime anyway,
we fill the time and millitm fields manually.

On android, the sys/timeb.h header has been removed in the next
platform version (currently available in NDK r10 in the "L preview"
release), since this header and the ftime function have been removed
from the POSIX 2008 specification.

This fixes compilation when targeting the Android "L preview" release,
even when targeting the 32 bit ABIs.
Add support for building for the new 64 bit android ABIs

This requires the new NDK r10.

The mips64 ABI is handled by the default case, there's no need to
add custom code for that in platform-android.mk.
Fix incorrect indentation in build/platform-android.mk
Merge pull request #1185 from mstorsjo/android-l-preview

Fix building for the Android L preview release, add support for new 64 bit Android ABIs