Move IGF temporary buffers out of the highest stack

• Related issues: #2428 (closed)
• Requested reviewers:

Reason why this change is needed

• Move IGF temporary buffers out of the highest stack

Description of the change

• Move IGF temporary buffers out of the highest stack in function stereo_mdct_core_enc_fx().
• The encoder's highest stack is reduced by 16,080 bytes.

Affected operating points

• All is BE.

Closes #2428 (closed)