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Add a new NDK-only ABI armeabi-v7a-hard in order to solve
-mfloat-abi=hard issue.

1. Note that this is NDK-only ABI.  No known devices
   support armeabi-v7a-hard, and they can't w/o breaking existing float-abi=softfp
   code
2. APP_ABI=armeabi-v7a-hard still installs libraries in libs/armeabi-v7a.
   As a result, armeabi-v7a-hard and armeabi-v7a can't co-exist ("make"
   will ignore one of the "install"), unless when _NDK_TESTING_ALL_=yes for
   internal testing (where lib is installed to libs/armeabi-v7a-hard)
3. Both GCC 4.6/4.8 and Clang3.3/3.4 has additional multilib
   option "-mfloat-abi=hard" to look for libraries in /hard
   sub-directory.
     5be615df32ce970fcccda93cf577e9ce35b1d397 (gcc4.6,4.8)
     7e8798057dc5f28b44b5608fd98fff1cba6909a2 (clang3.4)
     deedd8557b7d82c6931b6942bd39a00127780835 (clang3.3)
4. APP_ABI=all still expand to armeabi,armeabi-v7a,x86,mips

Change-Id: I08c26bfa6580c80260185c2fef398848aebd197d