linux, android: Add a generic definition for XCASE

[3v1n0]

Description

XCASE it's defined for all the architectures, although this is not happening currently.

The only arch definition that has a different value is powerpc

Sources

• Linux: https://github.com/search?q=repo%3Atorvalds%2Flinux%20XCASE&type=code
• glibc: https://github.com/search?q=repo%3Abminor%2Fglibc%20XCASE&type=code
• musl: https://github.com/search?q=repo%3Abminor%2Fmusl%20XCASE&type=code
• Android: https://cs.android.com/search?q=XCASE&ss=android%2Fplatform%2Fsuperproject%2Fmain

Checklist

• Relevant tests in libc-test/semver have been updated
• No placeholder or unstable values like *LAST or *MAX are
  included (see #3131)
• Tested locally (cd libc-test && cargo test --target mytarget);
  especially relevant for platforms that may not be checked in CI

[rustbot]

Some changes occurred in the Android module

cc @maurer