redefine _Float16 as __fp16 for x86[_64] to allow removing workaround from inhouse clang

At present, the tests might not actually use _Float16 (the generated
tests that are incorporated into the functional tests are built in
frameworks/ml rather than in hardware/interfaces); but the makefiles
are changed anyway, in case _Float16 is added in the future.

Bug: 138709788

Test: for (walleye, aosp_x86 [on emulator], aosp_x86_64 [on emulator])
        Use clang where _Float16 is disabled for x86[_64]
        $ cd $ANDROID_BUILD_TOP/neuralnetworks/1.0/vts/functional
        inspect build commands for tests as to whether or not
          expected options related to redefinition are present
        $ cd $ANDROID_BUILD_TOP/neuralnetworks/1.1/vts/functional
        inspect build commands for tests as to whether or not
          expected options related to redefinition are present

Change-Id: I9225b228947696d249771b3a35b1f82de9c26b6e
(cherry picked from commit 2452ee5a)