microblaze: fix endian handling

Building an allmodconfig kernel fails horribly because of
endian mismatch. It turns out that the -mlittle-endian
switch was not honored at all as we were using the wrong
Kconfig symbol and failing to apply CPUFLAGS to the CFLAGS.
Finally, the linker flags did not get set right.

This addresses all three of those issues, which now lets
me build both big-endian and little-endian kernels for
testing.

Fixes: 428dbf15 ("arch: change default endian for microblaze")
Fixes: 206d3642 ("arch/microblaze: add choice for endianness and update Makefile")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>