microblaze: Change section flags for noMMU

All files which uses user unified macros from uaccess.h
(get_user/put_user/clear_user/copy_tofrom_user/
strnlen_user and strncpy_user) generate this
warning messages:
Assembler messages:
Warning: ignoring changed section attributes for .discard

Setting up discard executable section flang for __EX_TABLE_SECTION
macro removed all these warnings.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>