gcc won't build i386 with our cctools assembler hack

[Ken Cunningham]

Iain's fix changes the assembly output to succeed if the assembler detected during the i386 gcc build is clang, but this is not an on-the-fly change, rather a configure-at-build change... so for our purposes, we will still need to force the older gas behaviour when gcc is built i386, so gcc will still work if clang 5+ is not installed.

I have sorted out how to do that (gcc does not make it trivial to set envvars) and it fixes the i386 builds if clang 5+ is installed.

As usual, I'll be testing it for a while prior to pushing it. Anyone interested in helping please volunteer and you can run the test suite.

This may now allow libgcc to move up on several systems as well, as it was only the i386 build holding it back, however there are reports of bootstrapping troubles where the current  gcc9/libgcc9 might no longer build with gcc-4.2...so there's that to review.

K