build: move rtc_prefer_fixed_point
build: move rtc_prefer_fixed_point

making it more visible where fixed point is preferred

BUG=None

Change-Id: I72c135ebea9c67739fc9c1747433f1a574319d39
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/183980
Reviewed-by: Mirko Bonadei <d2c43c210eae6feef04f53bae50885e8152edcca@webrtc.org>
Commit-Queue: Philipp Hancke <563b103f57dc2044b12822c2a7559aa15cb1c987@googlemail.com>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#32081}

TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc