cross/gcc-*: upgrade to 10.3.1_git20210921-r1
Building packages currently gives errors like:
ERROR: unable to select packages:
libstdc++-10.3.1_git20210921-r1:
breaks: g++-armv7-10.3.1_git20210625-r1[libstdc++=10.3.1_git20210625-r1]
satisfies: .pmbootstrap-20210921.191511[libstdc++]
gcc-10.3.1_git20210921-r1[so:libstdc++.so.6]
binutils-2.37-r1[so:libstdc++.so.6]
binutils-armv7-2.35.2-r2[so:libstdc++.so.6]
ccache-4.3-r0[so:libstdc++.so.6]
gcc-10.3.1_git20210921-r1:
breaks: g++-armv7-10.3.1_git20210625-r1[gcc=10.3.1_git20210625-r1]
satisfies: .pmbootstrap-20210921.191511[gcc]