Specifies that the PRU CTABLE entry given by index has the value
constant_address. This enables GCC to emit LBCO/SBCO instructions
when the load/store address is known and can be addressed with some CTABLE
entry. For example:
/* will compile to "sbco Rx, 2, 0x10, 4" */
#pragma ctable_entry 2 0x4802a000
*(unsigned int *)0x4802a010 = val;