OK, -fno-common seems to have fixed the alignment, from the map file,
Module: K:\work\ffmpeg\src\ffmpeg.obj\libavcodec\fft_fixed.o(src\libavcodec\fft_fixed.c)
0001:001fbc83 _ff_init_ff_cos_tabs_fixed
0001:001fc050 _ff_cos_tabs_fixed
0001:001fbd3c _ff_fft_init_fixed
0001:001fbf90 _ff_fft_end_fixed
0002:001cc900 _ff_cos_131072_fixed
0002:0020c8e0 _ff_cos_16_fixed
0002:0020c8c0 _ff_cos_32_fixed
0002:0020c880 _ff_cos_64_fixed
0002:0020c800 _ff_cos_128_fixed
0002:0020c700 _ff_cos_256_fixed
0002:0020c500 _ff_cos_512_fixed
0002:0020c100 _ff_cos_1024_fixed
0002:0020b900 _ff_cos_2048_fixed
0002:0020a900 _ff_cos_4096_fixed
0002:00208900 _ff_cos_8192_fixed
0002:00204900 _ff_cos_16384_fixed
0002:001fc900 _ff_cos_32768_fixed
0002:001ec900 _ff_cos_65536_fixed
...
Module: K:\work\ffmpeg\src\ffmpeg.obj\libavcodec\fft_float.o(src\libavcodec\fft_float.c)
0001:002114e0 _ff_init_ff_cos_tabs
0001:00211870 _ff_cos_tabs
0001:00211561 _ff_fft_init
0001:002117bc _ff_fft_end
0002:0020c900 _ff_cos_131072
0002:0028c8c0 _ff_cos_16
0002:0028c880 _ff_cos_32
0002:0028c800 _ff_cos_64
...
Edit, BTW, GCC outputs aout