pru: bfd: Correct default to no execstack

Data and instruction memories are strictly separated, so it is not
possible to execute instructions from the stack memory on PRU.

I don't see any difference in testsuite results with or without this
change.

bfd/ChangeLog:

	* elf32-pru.c (elf_backend_default_execstack): Define as 0.

ld/ChangeLog:

	* testsuite/ld-elf/elf.exp (target_defaults_to_execstack):
	Return 0 for pru.

Signed-off-by: Dimitar Dimitrov <dimitar@dinux.eu>