Power10 compile issues with gcc-10.

gcc-10 doesn't seem to officially support power10

• https://gcc.gnu.org/onlinedocs/gcc-10.3.0/gcc/RS_002f6000-and-PowerPC-Options.html#RS_002f6000-and-PowerPC-Options

Though it does seem to work on some systems, on others the compiler reports that the builtins exist, but the assembler fails with

/tmp/cci27rsX.s: Assembler messages:
/tmp/cci27rsX.s:153632: Error: invalid machine `power10'
/tmp/cci27rsX.s:153956: Error: unrecognized opcode: `xxsetaccz'
/tmp/cci27rsX.s:153958: Error: unrecognized opcode: `xxmfacc'
/tmp/cci27rsX.s:153959: Error: unrecognized opcode: `stxvp'
...

This happens, for example, using the docker ubuntu:20.04 image (both on a ppc64le machine, and on an x86-64 machine cross-compiling via powerpc64le-linux-gnu-g++-10).

To reproduce:

docker run -it ubuntu:20.04

export DEBIAN_FRONTEND=noninteractive
apt update
apt install -y git g++-10-powerpc64le-linux-gnu
git clone https://gitlab.com/libeigen/eigen.git
powerpc64le-linux-gnu-g++-10 -mvsx -I eigen/ -DEIGEN_TEST_PART_2=1 -DEIGEN_TEST_MAX_SIZE=10 eigen/test/random_matrix.cpp -o random_matrix