Android: Build webp plugin

We need to include the cpufeatures module in the build for
Android, and we need to make sure the neon files are compiled
with neon flags turned on. Since these are .c files, we
cannot use the regular simd.prf approach, which only supports
.cpp, so the compiler is basically copy-pasted from simd.prf,
but using QMAKE_CC instead of QMAKE_CXX.

[ChangeLog][Android] Added support for webp image format.

Change-Id: Id702b8202bee963d23beb02a7dc163756c8ca712
Task-number: QTBUG-38715
Reviewed-by: Liang Qi <liang.qi@theqtcompany.com>
Reviewed-by: aavit <eirik.aavitsland@theqtcompany.com>