treewide: remove IA32 support

[phip1611]

This is a prerequisite for the next OVMF bump (#1970). EDK2 removed IA32 support
since the 202508 release [0].

[0] tianocore/edk2@1fb88ff

Checklist

• Sensible git history (for example, squash "typo" or "fix" commits). See the Rewriting History guide for help.
• Update the changelog (if necessary)

[nicholasbishop]

I don't think removing IA32 support is our only option. I see a few possibilities here:

1. The older ovmf-prebuilt release edk2-stable202508-r1 still has IA32, and we could download that for our IA32 tests, while still using a newer release for x64/aarch64.
2. We could opt for an intermediate level of testing: still build IA32 in CI, but don't run the VM tests.
3. Or, we could decide that fully removing IA32 is the right option given that it was always a pretty niche target and that hardware is now pretty old.

As long as the maintenance costs aren't too high, my instinct would be to opt for 1. Although I'm no longer involved with ChromeOS Flex, I know that the bootloader is still building both x64 and ia32 variants. The usage of the ia32 variant is low, but not zero.

[phip1611]

I decided to solve this in #1970 in a pretty simple way. We fix OVMF for IA32 only to the latest stable release. The code change is pretty simple. All future code that needs newer OVMF just uses #[cfg(not(target_arch = "x86"))]