Rollup merge of rust-lang#126039 - dpaoliello:arm64ecbuild, r=davidtwco

Promote `arm64ec-pc-windows-msvc` to tier 2

MCP: <rust-lang/compiler-team#746>

* Update platform support docs
* Add `arm64ec-pc-windows-msvc` as a target to the existing AArch64 Windows build in CI.
* Fix docs build break.
* Add `arm64ec-pc-windows-msvc` to build manifest.

CI build (succeeded, but upload to S3 failed): <https://github.com/rust-lang/rust/actions/runs/9388227822/job/25853013083?pr=126039>

std/src/os/linux/raw.rs

@@ -244,7 +244,11 @@ mod arch {
     pub use libc::{blkcnt_t, blksize_t, ino_t, nlink_t, off_t, stat, time_t};
 }
 
-#[cfg(target_arch = "aarch64")]
+#[cfg(any(
+    target_arch = "aarch64",
+    // Arm64EC is Windows-only, but docs are always build as Linux, so re-use AArch64 for Arm64EC.
+    all(doc, target_arch = "arm64ec")
+))]
 mod arch {
     use crate::os::raw::{c_int, c_long};