Buildroot does not automatically add the needed CFLAGS and LDFLAGS to use the adequate multilib variant, because those CFLAGS and LDFLAGS are very toolchain-specific and architecture-specific. So it is the user's job to add explictly those flags.

I have posted a patch, http://lists.busybox.net/pipermail/buildroot/2012-January/049556.html, which adds some help text on the different external toolchains about their respective multilib variants and how to select between them. Hopefully, this patch should be merged for 2012.02.

Created attachment 4040 [details]
Patch to properly create symlinks when ARCH_SUBDIR is not a single directory.

(In reply to comment #1)
> Buildroot does not automatically add the needed CFLAGS and LDFLAGS to use the
> adequate multilib variant, because those CFLAGS and LDFLAGS are very
> toolchain-specific and architecture-specific. So it is the user's job to add
> explictly those flags.
> 
> I have posted a patch,
> http://lists.busybox.net/pipermail/buildroot/2012-January/049556.html, which
> adds some help text on the different external toolchains about their respective
> multilib variants and how to select between them. Hopefully, this patch should
> be merged for 2012.02.

(I'm sorry I posted a patch before the explanation)

I don't think this is enough (it didn't work for me).
The TOOLCHAIN_EXTERNAL_CFLAGS in toolchain/toolchain-external/ext-tool.mk does not take into account BR2_TARGET_OPTIMIZATION.
So, in the end it always copies the big-endian libc/ from multilib.

From toolchain/toolchain-external/ext-tool.mk:

# march/mtune/floating point mode needs to be passed to the external toolchain
# to select the right multilib variant
ifneq ($(CC_TARGET_TUNE_),)
TOOLCHAIN_EXTERNAL_CFLAGS += -mtune=$(CC_TARGET_TUNE_)
TOOLCHAIN_EXTERNAL_WRAPPER_ARGS += -DBR_TUNE='"$(CC_TARGET_TUNE_)"'
endif
ifneq ($(CC_TARGET_ARCH_),)
TOOLCHAIN_EXTERNAL_CFLAGS += -march=$(CC_TARGET_ARCH_)
TOOLCHAIN_EXTERNAL_WRAPPER_ARGS += -DBR_ARCH='"$(CC_TARGET_ARCH_)"'
endif
ifneq ($(CC_TARGET_CPU_),)
TOOLCHAIN_EXTERNAL_CFLAGS += -mcpu=$(CC_TARGET_CPU_)
TOOLCHAIN_EXTERNAL_WRAPPER_ARGS += -DBR_CPU='"$(CC_TARGET_CPU_)"'
endif
ifneq ($(CC_TARGET_ABI_),)
TOOLCHAIN_EXTERNAL_CFLAGS += -mabi=$(CC_TARGET_ABI_)
TOOLCHAIN_EXTERNAL_WRAPPER_ARGS += -DBR_ABI='"$(CC_TARGET_ABI_)"'
endif

ifeq ($(BR2_SOFT_FLOAT),y)
TOOLCHAIN_EXTERNAL_CFLAGS += -msoft-float
TOOLCHAIN_EXTERNAL_WRAPPER_ARGS += -DBR_SOFTFLOAT=1
endif

ifeq ($(BR2_VFP_FLOAT),y)
TOOLCHAIN_EXTERNAL_CFLAGS += -mfpu=vfp
TOOLCHAIN_EXTERNAL_WRAPPER_ARGS += -DBR_VFPFLOAT=1
endif

...

        $(Q)SYSROOT_DIR=`$(TOOLCHAIN_EXTERNAL_CC) -print-sysroot 2>/dev/null` ; \

...

        ARCH_SUBDIR=`$(TOOLCHAIN_EXTERNAL_CC) $(TOOLCHAIN_EXTERNAL_CFLAGS) -print-multi-directory` ; \
        ARCH_SYSROOT_DIR=$${SYSROOT_DIR}/$${ARCH_SUBDIR} ; \

...

        echo "Copy external toolchain sysroot to staging..." ; \
        $(call copy_toolchain_sysroot,$${SYSROOT_DIR},$${ARCH_SYSROOT_DIR},$${ARCH_SUBDIR}) ; \


I have selected the following in menuconfig (I've also selected soft-float):

BR2_TARGET_OPTIMIZATION="-pipe -EL -march=m14kc -mmcu"
BR2_TARGET_LDFLAGS="-EL"


I attempted to fix the problem by passing $(TARGET_LDFLAGS) in ARCH_SUBDIR:

        ARCH_SUBDIR=`$(TOOLCHAIN_EXTERNAL_CC) $(TOOLCHAIN_EXTERNAL_CFLAGS) $(TARGET_LDFLAGS) -print-multi-directory` ; \

After this change it seems that the little-endian version of libc is copied in buildroot's sysroot, but I still get errors about crt1.o and crti.o (ld can't find them).

This happens because in my case ARCH_SUBDIR=soft-float/el .
copy_toolchain_sysroot doesn't handle the case where ARCH_SUBDIR is not a single directory:

ln: failed to create symbolic link `/home/iordanidis/src/buildroot/buildroot/output/host/usr/mipsel-unknown-linux-gnu/sysroot/soft-float/el': No such file or directory

I corrected this by slightly modifying copy_toolchain_sysroot. After that, busybox was built correctly with -EL -soft-float.