Refactor common_audio/signal_processing: Removed usage of trivial macro WEBRTC_SPL_MUL_16_16

The macro is in C defined as
#define WEBRTC_SPL_MUL_16_16(a, b) \
((int32_t) (((int16_t)(a)) * ((int16_t)(b))))
(For definitions on ARMv7 and MIPS, see common_audio/signal_processing/include/spl_inl_{armv7,mips}.h)

Also includes
- style changes
- replaced pointer operations with direct element access

BUG=3348,3353
TESTED=locally on Linux and trybots
R=kwiberg@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/48949004

Cr-Commit-Position: refs/heads/master@{#9075}
10 files changed