Enable RISC-V bitmanip extensions for OpenTitan

Ibex in OpenTitan supports the bitmanip extensions in addition to IMC.
The ratified set of these are supported upstream in LLVM, but not
exposed in any of the default Rust targets. We can manually enable them
by adding this flag.

boards/cargo/riscv_bitmanip.toml

@@ -0,0 +1,11 @@
+# Licensed under the Apache License, Version 2.0 or the MIT License.
+# SPDX-License-Identifier: Apache-2.0 OR MIT
+# Copyright Tock Contributors 2024.
+
+# bitmanip instruction support for RISC-V.
+
+[build]
+rustflags = [
+  # These are the ratified bitmanip extensions supported by LLVM.
+  "-Ctarget-feature=+zba,+zbb,+zbc,+zbs",
+]

boards/opentitan/earlgrey-cw310/.cargo/config.toml

@@ -6,6 +6,7 @@ include = [
   "../../../cargo/tock_flags.toml",
   "../../../cargo/unstable_flags.toml",
   "../../../cargo/riscv_flags.toml",
+  "../../../cargo/riscv_bitmanip.toml",
   "../../../cargo/virtual_function_elimination.toml",
   "../../../cargo/panic_abort_tests.toml",
 ]