Build with bazel on darwin-arm64

[mcevoypeter]

Build the urbit binary on darwin-arm64 (i.e. Apple Silicon machines running macOS). This will build off the work done in #2. The steps required:

• Tweak third-party dependency bazel build rules in bazel/third_party/ to build on darwin-arm64 platforms.
• Move the platform-specific third-party dependency patches in pkg/urbit/compat/m1brew/ to the appropriate dependency directory in bazel/third_party/; verify the patches are still correct in the context of the dependency versions we're using; and apply the patches to the dependency buidls in WORKSPACE.bazel using select().
• Incorporate the compilation flags in pkg/urbit/compat/m1brew/compat.mk into the appropriate build rule(s) in pkg/.
• Tweak the bazel build rules for all packages in pkg/ to build on darwin-arm64 platforms.

[matthew-levan]

Notes from my conversation with @mcevoypeter today:

1. ARM chips don't support Intel SSE, which at least h2o depends on.
   a. ARM chips support NEON instead.
   b. We can replace SSE headers with sse2neon.h in our ARM-specific patches.
2. Our platform-specific patching strategies will include:
   a. For build-system patches: incorporate Makefile patches to cc_library rules in third-party <dependency>.BUILD files.
   b. For source code patches: use environment variables (i.e., URBIT_RUNTIME_PLATFORM_DARWIN, etc.) and #if defined(VAR) to gate platform-specific code blocks in source files, and set the appropriate environment variables using select() calls in our third-party <dependency>.BUILD files to "select" the correct code blocks.

TODOs:

• Implement updated patching strategies with:
  • murmur3
  • softfloat
  • lmdb
  • h2o
• Document patching strategies more thoroughly in the top-level comment of WORKSPACE.bazel

[mcevoypeter]

Closing now that #13 is merged.