AECM: MIPS: Use uintptr_t for pointer arithmetic

Trying to compile the MIPS-specific AECM audio processing file for
mips64el on Debian results in the following errors:

  ../webrtc/modules/audio_processing/aecm/aecm_core_mips.cc: In function ‘int webrtc::WebRtcAecm_ProcessBlock(AecmCore*, const int16_t*, const int16_t*, const int16_t*, int16_t*)’:
  ../webrtc/modules/audio_processing/aecm/aecm_core_mips.cc:955:30: error: cast from ‘int16_t*’ {aka ‘short int*’} to ‘uint32_t’ {aka ‘unsigned int’} loses precision [-fpermissive]
    955 |   int16_t* fft = (int16_t*)(((uint32_t)fft_buf + 31) & ~31);
        |                              ^~~~~~~~~~~~~~~~~
  ../webrtc/modules/audio_processing/aecm/aecm_core_mips.cc:955:18: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
    955 |   int16_t* fft = (int16_t*)(((uint32_t)fft_buf + 31) & ~31);
        |                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  ../webrtc/modules/audio_processing/aecm/aecm_core_mips.cc:956:36: error: cast from ‘int32_t*’ {aka ‘int*’} to ‘uint32_t’ {aka ‘unsigned int’} loses precision [-fpermissive]
    956 |   int32_t* echoEst32 = (int32_t*)(((uint32_t)echoEst32_buf + 31) & ~31);
        |                                    ^~~~~~~~~~~~~~~~~~~~~~~
  ../webrtc/modules/audio_processing/aecm/aecm_core_mips.cc:956:24: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
    956 |   int32_t* echoEst32 = (int32_t*)(((uint32_t)echoEst32_buf + 31) & ~31);
        |                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  ../webrtc/modules/audio_processing/aecm/aecm_core_mips.cc:957:40: error: cast from ‘int32_t*’ {aka ‘int*’} to ‘uint32_t’ {aka ‘unsigned int’} loses precision [-fpermissive]
    957 |   ComplexInt16* dfw = (ComplexInt16*)(((uint32_t)dfw_buf + 31) & ~31);
        |                                        ^~~~~~~~~~~~~~~~~
  ../webrtc/modules/audio_processing/aecm/aecm_core_mips.cc:957:23: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
    957 |   ComplexInt16* dfw = (ComplexInt16*)(((uint32_t)dfw_buf + 31) & ~31);
        |                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  ../webrtc/modules/audio_processing/aecm/aecm_core_mips.cc:958:40: error: cast from ‘int32_t*’ {aka ‘int*’} to ‘uint32_t’ {aka ‘unsigned int’} loses precision [-fpermissive]
    958 |   ComplexInt16* efw = (ComplexInt16*)(((uint32_t)efw_buf + 31) & ~31);
        |                                        ^~~~~~~~~~~~~~~~~
  ../webrtc/modules/audio_processing/aecm/aecm_core_mips.cc:958:23: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
    958 |   ComplexInt16* efw = (ComplexInt16*)(((uint32_t)efw_buf + 31) & ~31);
        |                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Presumably, this file was written for 32-bit MIPS so the author used
uint32_t to do pointer arithmetic over these arrays. Fix the errors by
using uintptr_t to work with pointers.

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>