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]

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[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

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[tropicaaal]

Not a blocker, but it might be a good idea for this to be elaborated on wrt. why we changed to thumb.

View changes since the review

[tropicaaal]

LGTM

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