re-enable linux arm64 builds (#1060)

Fix #1011 by avoiding
the use of `crate-type = ["cdylib"]` in native builds.

It was causing some linker errors when cross-compiling from x64-linux to
aarch64-linux, I believe the root cause is
rust-lang/cargo#4133 i.e. cargo failing to
select the right target linker during cross builds.

The `cdylib` crate type is only relevant when building wasms, so this
change just lowers that functionality into the specific commands that
build wasms.

---------

Co-authored-by: Leigh McCulloch <351529+leighmcculloch@users.noreply.github.com>

.github/workflows/rust.yml

@@ -61,10 +61,14 @@ jobs:
         sys:
         - os: ubuntu-latest
           target: x86_64-unknown-linux-gnu
-        # TODO: Figure out why builds are broken for linux arm64 and reenable.
-        # https://github.com/stellar/rs-soroban-sdk/issues/1011
-        # - os: ubuntu-latest
-        #   target: aarch64-unknown-linux-gnu
+        - os: ubuntu-latest
+          target: aarch64-unknown-linux-gnu
+          # Fix #1011 by avoiding the use of crate-type = ["cdylib"] in native
+          # builds that cross-compile. It causes linker errors. The root cause
+          # is rust-lang/cargo#4133. This workaround uses rustc to build for a
+          # specific crate-type, lib, such that the Cargo.toml crate-type is
+          # ignored.
+          cdylib-cross-compile-workaround: true
         - os: macos-latest
           target: x86_64-apple-darwin
         - os: macos-latest
@@ -85,7 +89,10 @@ jobs:
         name: cargo-hack
         version: 0.5.16
     - run: cargo-hack hack build --target wasm32-unknown-unknown --profile release
-    - run: cargo-hack hack --feature-powerset --exclude-features docs build --target ${{ matrix.sys.target }}
+    - if: "!matrix.sys.cdylib-cross-compile-workaround"
+      run: cargo-hack hack --feature-powerset --exclude-features docs build --target ${{ matrix.sys.target }}
+    - if: "matrix.sys.cdylib-cross-compile-workaround"
+      run: cargo-hack hack --feature-powerset --exclude-features docs rustc --workspace --exclude soroban-sdk-macros --crate-type lib --target ${{ matrix.sys.target }}
     - if: startsWith(matrix.sys.target, 'x86_64')
       run: cargo-hack hack --feature-powerset --ignore-unknown-features --features testutils --exclude-features docs test --target ${{ matrix.sys.target }}