Add support for armeabi-v7a to the NDK.

Add support for armeabi-v7a to the NDK.

This patch modifies the NDK build scripts to support the 'armeabi-v7a' ABI.
(For the record, it corresponds to Thumb-2 + FPU support, to speed up native
code on certain devices like the Droid).

To build for this ABI, the Application.mk file should use a line like:

  APP_ABI := armeabi-v7a

It is also possible to build for both 'armeabi' and 'armeabi-v7a' by using:

  APP_ABI := armeabi armeabi-v7a

This will result in the generation of two distinct shared libraries that both
will be copied to the final application package.

This is dependent on having GCC 4.4.0 prebuilt binaries under
build/prebuilt/<host>/arm-eabi-4.4.0, since gcc 4.2.1 does not support
this new ABI.

Note that this also changes the NDK to use gcc 4.4.0 by default, unless
the user defines NDK_TOOLCHAIN to 'arm-eabi-4.2.1' in its environment to
switch back to the previous one.