bash unconditionally builds locale.c which depends on mblen since version 5.0 and https://github.com/bminor/bash/commit/d233b485e83c3a784b803fb894280773f16f2deb

This results in the following build failure if wchar is not available:

/home/buildroot/autobuild/run/instance-0/output-1/host/bin/microblazeel-buildroot-linux-uclibc-gcc -L./builtins -L/home/buildroot/autobuild/run/instance-0/output-1/host/microblazeel-buildroot-linux-uclibc/sysroot/lib -L/home/buildroot/autobuild/run/instance-0/output-1/host/microblazeel-buildroot-linux-uclibc/sysroot/lib -L./lib/glob -L./lib/tilde  -L./lib/sh  -rdynamic -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64  -O2   -Wno-parentheses -Wno-format-security   -o bash shell.o eval.o y.tab.o general.o make_cmd.o print_cmd.o  dispose_cmd.o execute_cmd.o variables.o copy_cmd.o error.o expr.o flags.o jobs.o subst.o hashcmd.o hashlib.o mailcheck.o trap.o input.o unwind_prot.o pathexp.o sig.o test.o version.o alias.o array.o arrayfunc.o assoc.o braces.o bracecomp.o bashhist.o bashline.o  list.o stringlib.o locale.o findcmd.o redir.o pcomplete.o pcomplib.o syntax.o xmalloc.o signames.o -lbuiltins -lglob -lsh -lreadline -lhistory -lcurses -ltilde     -ldl
/home/buildroot/autobuild/run/instance-0/output-1/host/lib/gcc/microblazeel-buildroot-linux-uclibc/8.3.0/../../../../microblazeel-buildroot-linux-uclibc/bin/ld: locale.o: in function `set_default_locale':
(.text+0x260): undefined reference to `mblen'

To fix this issue, don't use mblen if HANDLE_MULTIBYTE is not defined, an other possibility would be to use MBLEN wrapper defined in shmbutil.h

More information can be found here:
http://autobuild.buildroot.org/results/298fb9c785e137bff432dd304eb56986e54ce3ed