bufferpool: Address ABI breakage for x86_64 targets

FMQ function instantiations should also be guarded against x86_64 along
with aarch64 to prevent ABI breakage.

Test: check `compatibility_status` on abidiff after full build
Test: out/soong/.intermediates/frameworks/av/media/bufferpool/2.0/libstagefright_bufferpool@2.0/android_x86_64_vendor_shared/libstagefright_bufferpool@2.0.so.abidiff
Signed-off-by: Daniel Jacob Chittoor <djchittoor@murena.io>