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/third_party/gperftools/BUILD b/third_party/gperftools/BUILD
index 7dd86dc..2022ee6 100644
--- a/third_party/gperftools/BUILD
+++ b/third_party/gperftools/BUILD
@@ -102,12 +102,18 @@
         "-DPRIuS=\\\"lu\\\"",
         "-DPRIxS=\\\"lx\\\"",
     ],
-    "arm": [
+    "arm32": [
         "-DPC_FROM_UCONTEXT=uc_mcontext.arm_pc",
         "-DPRIdS=\\\"d\\\"",
         "-DPRIuS=\\\"u\\\"",
         "-DPRIxS=\\\"x\\\"",
     ],
+    "arm64": [
+        "-DPC_FROM_UCONTEXT=uc_mcontext.pc",
+        "-DPRIdS=\\\"ld\\\"",
+        "-DPRIuS=\\\"lu\\\"",
+        "-DPRIxS=\\\"lx\\\"",
+    ],
 }) + compiler_select({
     "clang": [
         "-Wno-unused-const-variable",