Test natively on AArch64, not emulated (#10306)

This commit updates the aarch64 tests to use the new `*-arm` images from
GitHub instead of running through QEMU emulation. This should be a bit
faster but primarily helps build confidence that it runs on real
hardware.

I'll note that release builds aren't updated at this time to run on
native hardware since that's moreso about glibc compatibility and it's a
bit easier to keep the setup we currently have for that. If `*-arm`
machines are noticably faster than the default x64 machines then we
could in theory move everything over to an aarch64-based machine as
opposed to just the aarch64 build.

Author: alexcrichton

ci/build-test-matrix.js

@@ -95,12 +95,8 @@ const FULL_MATRIX = [
     "filter": "mingw-x64"
   },
   {
-    "os": ubuntu,
+    "os": ubuntu + '-arm',
     "target": "aarch64-unknown-linux-gnu",
-    "gcc_package": "gcc-aarch64-linux-gnu",
-    "gcc": "aarch64-linux-gnu-gcc",
-    "qemu": "qemu-aarch64 -L /usr/aarch64-linux-gnu",
-    "qemu_target": "aarch64-linux-user",
     "name": "Test Linux arm64",
     "filter": "linux-arm64",
     "isa": "aarch64",