x/build: add a linux/i386 builder without `COMPAT_32BIT_TIME` Kconfig set

[Jorropo]

I am reviewing a CL https://go-review.googlesource.com/c/go/+/701615 that aims to fix a bug (#75133) where the go runtime stopped working on 32 bits linux systems unless a compatibility Kconfig flag is set.

Could we please get a builder where this flag is unset (COMPAT_32BIT_TIME=n) ?

[orangecms]

The other test case for the new change would be a pre-5.0 kernel, before that Kconfig option was introduced - does that exist?

[Jorropo]

I don't know but this wouldn't help here.

The problem of #75133 is that 5.1 removed the futex syscall unless COMPAT_32BIT_TIME is set.
versions before just have that syscall.

[orangecms]

Yea, the pre-5.0 test would be for the fallback to check that older systems do not break.
I've tested it in VMs, and it worked fine.