I'm trying to compile gcc-4.8.1 from the original source on CentOS 6.4 (32-bit).  I unpacked gcc and mpc-1.0.1, mpfr-3.1.2, gmp-5.1.2 (the latest released versions of each), made symlinks to the libs, and ran configure from a sibling directory, like this:

tar xf gcc-4.8.1.tar.bz2 
...
mkdir gccbuild
cd gccbuild
../gcc-4.8.1/configure --prefix=/usr/local/gnu --enable-languages=c,c++,objc
make bootstrap

This went along fine for quite a while, but then aborted with (full command below; it's just whatever gcc's make does):
g++ -c  -DIN_GCC_FRONTEND -g -DIN_GCC [...] ../../gcc-4.8.1/gcc/cp/except.c
/tmp/ccKcKFPk.s: Assembler messages:
/tmp/ccKcKFPk.s:7332: Error: unrecognized symbol type "gnu_unique_object"
/tmp/ccKcKFPk.s:8051: Error: unrecognized symbol type "gnu_unique_object"
make[3]: *** [cp/except.o] Error 1
make[3]: Leaving directory `/u/local/gnu/src/gccb/gcc'

Looking at the configure output, I see:
  checking assembler for gnu_unique_object... no

So, on the face of it, it seems that cp/except.c shouldn't be trying to use this feature?

FWIW, /usr/bin/as --version on this system (CentOS 6.4, fully patched) says:
  GNU assembler version 2.20.51.0.2-5.36.el6 20100205

Thanks,
Karl

For the record, here is the full compile line that failed.
g++ -c  -DIN_GCC_FRONTEND -g -DIN_GCC   -fno-exceptions -fno-rtti -fasynchronou\
s-unwind-tables -W -Wall -Wwrite-strings -Wcast-qual -Wmissing-format-attribute\
 -pedantic -Wno-long-long -Wno-variadic-macros -Wno-overlength-strings -fno-com\
mon  -DHAVE_CONFIG_H -I. -Icp -I../../gcc-4.8.1/gcc -I../../gcc-4.8.1/gcc/cp -I\
../../gcc-4.8.1/gcc/../include -I../../gcc-4.8.1/gcc/../libcpp/include -I/usr/l\
ocal/gnu/src/gccb/./gmp -I/usr/local/gnu/src/gcc-4.8.1/gmp -I/usr/local/gnu/src\
/gccb/./mpfr/src -I/usr/local/gnu/src/gcc-4.8.1/mpfr/src -I/usr/local/gnu/src/g\
cc-4.8.1/mpc/src  -I../../gcc-4.8.1/gcc/../libdecnumber -I../../gcc-4.8.1/gcc/.\
./libdecnumber/bid -I../libdecnumber -I../../gcc-4.8.1/gcc/../libbacktrace    .\
./../gcc-4.8.1/gcc/cp/except.c -o cp/except.o