Put dispatch_tab in the rodata section for macho64.

This fixes linking shared libavcodec, since the linker doesn't allow
text relocations in shared libraries under Darwin/x86_64.

Based on a patch by Art Clarke (aclarke xuggle com)

Originally committed as revision 17197 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/x86/fft_mmx.asm

@@ -446,9 +446,15 @@ fft %+ n %+ %3%2:
 %endrep
 %undef n
 
+%ifidn __OUTPUT_FORMAT__,macho64
+section .rodata
+%endif
+
 align 8
 dispatch_tab%3%2: pointer list_of_fft
 
+section .text
+
 ; On x86_32, this function does the register saving and restoring for all of fft.
 ; The others pass args in registers and don't spill anything.
 cglobal fft_dispatch%3%2, 2,5,0, z, nbits