[Bazel] Add support for targeting Linux riscv64

[GMNGeoffrey]

The CI is still using Bazel 4.0, so it doesn't have linux_riscv64

[GMNGeoffrey]

To add it, we'd need to upgrade to 5.x, which is nontrivial because of the current use of remote execution, unfortunately. We're looking into moving these builders over to building locally, but when I tried it out I ran into some issues: https://github.com/google/llvm-premerge-checks/pull/394. @goncharov is working on making this all better supported, I believe

utils/bazel/llvm-project-overlay/llvm/config.bzl

 
 # TODO: We should split out host vs. target here.
 llvm_config_defines = os_defines + select({
     "@bazel_tools//src/conditions:windows": native_arch_defines("X86", "x86_64-pc-win32"),
     "@bazel_tools//src/conditions:darwin_arm64": native_arch_defines("AArch64", "arm64-apple-darwin"),
     "@bazel_tools//src/conditions:darwin_x86_64": native_arch_defines("X86", "x86_64-unknown-darwin"),
     "@bazel_tools//src/conditions:linux_aarch64": native_arch_defines("AArch64", "aarch64-unknown-linux-gnu"),
     "@bazel_tools//src/conditions:linux_ppc64le": native_arch_defines("PowerPC", "powerpc64le-unknown-linux-gnu"),
+    "@bazel_tools//src/conditions:linux_riscv64": native_arch_defines("RISCV", "riscv64-unknown-linux-gnu"),
     "@bazel_tools//src/conditions:linux_s390x": native_arch_defines("SystemZ", "systemz-unknown-linux_gnu"),
     "//conditions:default": native_arch_defines("X86", "x86_64-unknown-linux-gnu"),
 }) + [
     # These shouldn't be needed by the C++11 standard, but are for some
     # platforms (e.g. glibc < 2.18. See
     # https://sourceware.org/bugzilla/show_bug.cgi?id=15366). These are also
     # included unconditionally in the CMake build:
     # https://github.com/llvm/llvm-project/blob/cd0dd8ece8e/llvm/cmake/modules/HandleLLVMOptions.cmake#L907-L909
     "__STDC_LIMIT_MACROS",
     "__STDC_CONSTANT_MACROS",
     "__STDC_FORMAT_MACROS",
 ]