[libc] Include CPU model in overlay CI sccache key (#196477)

[libc] Include CPU model in overlay CI sccache key

The overlay CI compiles opt_host memory tests with `-march=native`,
which generates object files specific to the runner CPU model. sccache
treats `-march=native` as a literal string in its hash key, so cached
`.o` files compiled on one CPU model get served to runners with a
different CPU. When the cached binary uses instructions the current CPU
lacks, the test crashes with SIGILL.

## Symptoms

The `memcmp_opt_host`, `memmove_opt_host`, `memset_opt_host`,
`bcmp_opt_host`, and `bzero_opt_host` tests crash when SIMD code paths
are first exercised. Simple tests like `CmpZeroByte` pass because they
use small sizes that do not enter SIMD routines. The failures are fully
reproducible on reruns because the cache stays poisoned.

## Evidence

Three consecutive runs of the same fwide PR (#196157), same code:

| Run | Azure Region | Cache Hits | Cache Misses | Result |
|-----|-------------|-----------|-------------|--------|
|
[25512875679](https://github.com/llvm/llvm-project/actions/runs/25512875679/job/74876008545)
| westus3 | 9 | 5354 | PASS |
|
[25524024922](https://github.com/llvm/llvm-project/actions/runs/25524024922/job/74916241365)
| northcentralus | 5345 | 0 | CRASH |
|
[25524839613](https://github.com/llvm/llvm-project/actions/runs/25524839613/job/74965830435)
| westus | 5345 | 0 | CRASH |

The first run had a nearly empty cache and compiled everything locally
(0.17% hit rate). An intermediate [syscall-unistd
run](https://github.com/llvm/llvm-project/actions/runs/25517783708/job/74893495220)
in eastus then populated the cache with object files compiled for that
region's CPU. Subsequent runs on different hardware got 100% cache hits
and crashed because the cached `.o` files use instructions their CPUs
lack.

## Fix

Added a "Detect CPU model" step that reads the CPU model string from
`/proc/cpuinfo` (Linux) or `sysctl` (macOS) and appends it to the
sccache cache key. Runners with different CPUs now get separate cache
buckets.

Assisted-by: Automated tooling, human reviewed.