FFT: reduce stack and harmonize functions

• Related issues: #2365
• Requested reviewers:

Reason why this change is needed

• Removal of duplicated functions and reduction of highest stack

Description of the change

• OPTIMIZE_FFT_STACK
  • removal of intermediate FFT buffer from fft_fx(): 7,680 Bytes
  • move intermediate buffer out of the highest stack in ivas_mdct_core_whitening_enc_fx(): 9,600 Bytes
• HARMONIZE_DCT - removal of duplicated functions
• Saves:
  • 17,280 Bytes from highest stack (both encoder and decoder)
  • 8,000 Bytes of Table ROM
  • ~400 lines of source code

Affected operating points

• All is BE.