Upgrade to latest Rust nightly v1.75

[kevinaboos]

• Upgrade proc-macro2 versions used in uefi-builder tool to avoid this issue: Regression building proc-macro2 1.0.59 with nightly-2023-06-28 rust-lang/rust#113152

• Modify how the .theseus_merged special section is detected by the crate loading/linking code.

[tsoutsman]

Weird that the ovmf-prebuilt dependency was still in the lock file. Looks good other than clippy. Is there a reason we don't use THESEUS_MERGED_SEC_SHNDX any more?

[tsoutsman]

From Discord:

One new issue that I observed is that there are now GROUP sections (classified as COMDAT groups) for the rust_eh_personality sections (which are related to unwinding), but they only show up in aarch64 builds AFAICT.
I'm not sure if this is something that we need to be aware of; in the past I did not pay any attention to personality sections because we don't use that in our unwinder.
In any case, I just added a quick workaround to allow crate loading to work by ignoring the section.

[kevinaboos]

Is there a reason we don't use THESEUS_MERGED_SEC_SHNDX any more?

the existence of a GROUP section type may come before it, so I took a less fragile approach of looking for the .theseus_merged section anywhere before the .text* sections, since that is definitely under the control of our linker script.