mk: Request -march=i686 on i686 Linux

mk: Request -march=i686 on i686 Linux

Apparently the gcc on our dist bot is so old and/or obscure that the default
`-m32` switch doesn't think it can generate i686 code (or something like that).
The compiler-rt build system probes for the `__i686__` define in GCC to compile
for an i686 (vs i386) target, so this was failing on the bots.

This tweaks instead to pass `-march=i686` on i686-unknown-linux-gnu to C code to
ensure that we're compiling for i686 instead of i386. This should hopefully not
actually have an impact other than maybe doing some random optimization it
wasn't able to do so before. In theory this isn't making the target less
compatible as all Rust code is already compiled for i686.

Hopefully closes #34572