microblaze: disable microblaze64 gcc

This is because I don't intend to offer Zephyr support for microblaze64 yet
but also because the toolchain is buggy to the extent that it fails to
build.

Signed-off-by: Alp Sayin <alpsayin@gmail.com>

gcc/config/microblaze/t-microblaze

@@ -1,5 +1,5 @@
-MULTILIB_OPTIONS = m64 mxl-barrel-shift mlittle-endian mno-xl-soft-mul mxl-multiply-high
-MULTILIB_DIRNAMES = m64 bs le m mh
+MULTILIB_OPTIONS = mxl-barrel-shift mlittle-endian mno-xl-soft-mul mxl-multiply-high
+MULTILIB_DIRNAMES = bs le m mh
 MULTILIB_EXCEPTIONS = *m64/mxl-multiply-high mxl-multiply-high
 MULTILIB_EXCEPTIONS += *m64
 MULTILIB_EXCEPTIONS += *m64/mxl-barrel-shift