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when I sudo emerge -uDNav world, I got these errors.

checking for arm-linux-gnueabi-gcc... /var/tmp/portage/cross-arm-linux-gnueabi/gcc-4.7.3-r1/work/build/./gcc/xgcc -B/var/tmp/portage/cross-arm-linux-gnueabi/gcc-4.7.3-r1/work/build/./gcc/ -B/usr/arm-linux-gnueabi/bin/ -B/usr/arm-linux-gnueabi/lib/ -isystem /usr/arm-linux-gnueabi/include -isystem /usr/arm-linux-gnueabi/sys-include
checking for suffix of object files... configure: error: in `/var/tmp/portage/cross-arm-linux-gnueabi/gcc-4.7.3-r1/work/build/arm-linux-gnueabi/libgcc':
configure: error: cannot compute suffix of object files: cannot compile
See `config.log' for more details.
make[1]: *** [configure-target-libgcc] Error 1
make[1]: Leaving directory `/var/tmp/portage/cross-arm-linux-gnueabi/gcc-4.7.3-r1/work/build'
make: *** [all] Error 2
emake failed
 * ERROR: cross-arm-linux-gnueabi/gcc-4.7.3-r1::akoverlay failed (compile phase):
 *   emake failed with all
 *
 * Call stack:
 *     ebuild.sh, line   93:  Called src_compile
 *   environment, line 3746:  Called toolchain_src_compile
 *   environment, line 4465:  Called gcc_do_make
 *   environment, line 1913:  Called die
 * The specific snippet of code:
 *       emake LDFLAGS="${LDFLAGS}" STAGE1_CFLAGS="${STAGE1_CFLAGS}" LIBPATH="${LIBPATH}" BOOT_CFLAGS="${BOOT_CFLAGS}" ${GCC_MAKE_TARGET} || die "emake failed with ${GCC_MAKE_TARGET}";
 *
 * If you need support, post the output of `emerge --info '=cross-arm-linux-gnueabi/gcc-4.7.3-r1::akoverlay'`,
 * the complete build log and the output of `emerge -pqv '=cross-arm-linux-gnueabi/gcc-4.7.3-r1::akoverlay'`.
 *
 * Please include /var/tmp/portage/cross-arm-linux-gnueabi/gcc-4.7.3-r1/work/gcc-build-logs.tar.bz2 in your bug report.
 *
 * The complete build log is located at '/var/tmp/portage/cross-arm-linux-gnueabi/gcc-4.7.3-r1/temp/build.log'.
 * The ebuild environment file is located at '/var/tmp/portage/cross-arm-linux-gnueabi/gcc-4.7.3-r1/temp/environment'.
 * Working directory: '/var/tmp/portage/cross-arm-linux-gnueabi/gcc-4.7.3-r1/work/build'
 * S: '/var/tmp/portage/cross-arm-linux-gnueabi/gcc-4.7.3-r1/work/gcc-4.7.3'

>>> Failed to emerge cross-arm-linux-gnueabi/gcc-4.7.3-r1, Log file:

>>>  '/var/tmp/portage/cross-arm-linux-gnueabi/gcc-4.7.3-r1/temp/build.log'

 * Messages for package sys-apps/systemd-215-r3:

 * It's recommended to set an empty value to the following kernel config option:
 * CONFIG_UEVENT_HELPER_PATH="/sbin/udevadm"

 * Messages for package app-emulation/qemu-2.0.0-r1:

 *   You must enable VHOST_NET to have vhost-net support
 * Please check to make sure these options are set correctly.
 * Failure to do so may cause unexpected problems.

 * Messages for package cross-arm-linux-gnueabi/gcc-4.7.3-r1:

 * ERROR: cross-arm-linux-gnueabi/gcc-4.7.3-r1::akoverlay failed (compile phase):
 *   emake failed with all
 *
 * Call stack:
 *     ebuild.sh, line   93:  Called src_compile
 *   environment, line 3746:  Called toolchain_src_compile
 *   environment, line 4465:  Called gcc_do_make
 *   environment, line 1913:  Called die
 * The specific snippet of code:
 *       emake LDFLAGS="${LDFLAGS}" STAGE1_CFLAGS="${STAGE1_CFLAGS}" LIBPATH="${LIBPATH}" BOOT_CFLAGS="${BOOT_CFLAGS}" ${GCC_MAKE_TARGET} || die "emake failed with ${GCC_MAKE_TARGET}";
 *
 * If you need support, post the output of `emerge --info '=cross-arm-linux-gnueabi/gcc-4.7.3-r1::akoverlay'`,
 * the complete build log and the output of `emerge -pqv '=cross-arm-linux-gnueabi/gcc-4.7.3-r1::akoverlay'`.
 *
 * Please include /var/tmp/portage/cross-arm-linux-gnueabi/gcc-4.7.3-r1/work/gcc-build-logs.tar.bz2 in your bug report.
 *
 * The complete build log is located at '/var/tmp/portage/cross-arm-linux-gnueabi/gcc-4.7.3-r1/temp/build.log'.
 * The ebuild environment file is located at '/var/tmp/portage/cross-arm-linux-gnueabi/gcc-4.7.3-r1/temp/environment'.
 * Working directory: '/var/tmp/portage/cross-arm-linux-gnueabi/gcc-4.7.3-r1/work/build'
 * S: '/var/tmp/portage/cross-arm-linux-gnueabi/gcc-4.7.3-r1/work/gcc-4.7.3'