microblaze: drop/restrict override of .text, .data, and .bss

microblaze: drop/restrict override of .text, .data, and .bss

While only ELF is supported right now, (stub) code generally is in place
for the non-ELF case as well. Don't override .bss for ELF - that's
unlikely to be a good idea anyway and prevented the sub-section
specifier from being usable. Don't override .text and .data at all - for
.data and ELF for the same reason, while for .text and ELF obj-elf.c's is
all we need, and for (hypothetical) non-ELF read.c's identical handling
would have been invoked anyway.