libFDK: fix undefined operations [Werror=sequence-point]

This commit accomplishes two tasks:

1) Fixes undefined operations that can be seen as so:
    external/aac/libFDK/src/arm/qmf_arm.cpp: In function 'void qmfSynPrototypeFirSlot1_filter(FIXP_DBL*, FIXP_DBL*, const FIXP_DBL*, FIXP_DBL*, FIXP_DBL*, int)':
    external/aac/libFDK/src/arm/qmf_arm.cpp:485:40: error: operation on 'sta' may be undefined [-Werror=sequence-point]
          *sta++ = SMLAWT( sta[1], imag, B );  /* index=9...........319  */
                                            ^
    etc.

2) Removes compiler flags that are no longer needed.

Change-Id: I118404cbc8c261e6faba0c7e30ac7bb482f89d76