Rollup merge of #49731 - japaric:std-thumb-for-real, r=alexcrichton

add THUMB targets to rustup manifest

as instructed in #49563 (comment)

r? @alexcrichton

Author: kennytm

src/tools/build-manifest/src/main.rs

@@ -90,6 +90,10 @@ static TARGETS: &'static [&'static str] = &[
     "sparc-unknown-linux-gnu",
     "sparc64-unknown-linux-gnu",
     "sparcv9-sun-solaris",
+    "thumbv6m-none-eabi",
+    "thumbv7em-none-eabi",
+    "thumbv7em-none-eabihf",
+    "thumbv7m-none-eabi",
     "wasm32-unknown-emscripten",
     "wasm32-unknown-unknown",
     "x86_64-apple-darwin",