Emit thumb code on VEX V5

[lewisfm]

This PR switches the default codegen for the VEX V5 target to emit Thumb-2 instructions, allowing for smaller binary sizes (on the programs I tested this change on, I saw a ~20% size decrease on average). The target is renamed to have the thumb prefix instead of the arm prefix because of the updated instruction set.

Since VEXos starts all programs in Arm32 mode, the program entrypoint is explicitly compiled as ARM code and now transitions to Thumb mode when calling _start.

Users can still use the updated target in Arm32 mode by specifying -Ctarget-feature=-thumb-mode.

cc @tropicaaal @Gavin-Niederman

[rustbot]

Some changes occurred in src/doc/rustc/src/platform-support

cc @Noratrieb

These commits modify compiler targets.
(See the Target Tier Policy.)

[rustbot]

r? @Kivooeo

rustbot has assigned @Kivooeo.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: @ehuss, compiler
• @ehuss, compiler expanded to 76 candidates
• Random selection from 23 candidates

[rust-log-analyzer]

A job failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

  PR_CI_JOB: 1
  IMAGE: x86_64-gnu-llvm-21
##[endgroup]
    Updating crates.io index
error: failed to get `fnv` as a dependency of package `http v1.2.0`
    ... which satisfies dependency `http = "^1.1.0"` of package `ureq-proto v0.3.3`
    ... which satisfies dependency `ureq-proto = "^0.3.3"` of package `ureq v3.0.8`
    ... which satisfies dependency `ureq = "^3"` of package `citool v0.1.0 (/home/runner/work/rust/rust/src/ci/citool)`

Caused by:
  failed to load source for dependency `fnv`

Caused by:
  unable to update registry `crates-io`

Caused by:
  download of 3/f/fnv failed

Caused by:
  curl failed

Caused by:

[Gavin-Niederman]

LGTM

View changes since this review