Add arm64 support

This patch adds `--config=arm64` and `--cpu=arm64` flags for users to
compile for the 64-bit version of the Raspberry Pi.

The existing 32-bit ARM build should still work. This patch only adds
additional 64-bit capabilities. Future patches can remove 32-bit
support.

Eigen needed some tweaks because we have `LDBL_MANT_DIG` of 113 on
arm64. None of the code paths that used the parameters `U` and `V`
actually gets compiled in. Adding `[[maybe_unused]]` fixed the error.

Signed-off-by: Philipp Schrader <philipp.schrader@gmail.com>
Change-Id: Ibf919b2964fe971769449a1c556c13e3bf1d48e9
diff --git a/tools/BUILD b/tools/BUILD
index 2e470fa..a7d165d 100644
--- a/tools/BUILD
+++ b/tools/BUILD
@@ -47,11 +47,14 @@
 
 config_setting(
     name = "cpu_armhf",
-    constraint_values = ["@//tools/platforms/hardware:raspberry_pi"],
+    constraint_values = [
+        "@platforms//cpu:armv7",
+        "//tools/platforms/hardware:raspberry_pi",
+    ],
 )
 
 config_setting(
-    name = "cpu_aarch64",
+    name = "cpu_arm64",
     constraint_values = ["@platforms//cpu:arm64"],
 )