Add cheriot-baremetal toolchain support

This adds support for the cheriot-baremetal ABI intended for code
that runs before the CHERIoT loader. This can be selected using
--config=cheriot-baremetal on the cmd line.

Change-Id: I86a1c4b9b68292cba2708b3c4e50e456c35b87ff

.bazelrc

diff --git a/.bazelrc b/.bazelrc
index bf3b43b..daac0b0 100644
--- a/.bazelrc
+++ b/.bazelrc
@@ -28,6 +28,7 @@
 build:kelvin --platforms=//platforms/riscv32:kelvin
 build:sparrow --platforms=@matcha//platforms/riscv32:sparrow
 build:cheriot --platforms=@matcha//platforms/cheri/riscv32:cheriot
+build:cheriot-baremetal --platforms=@matcha//platforms/cheri/riscv32:cheriot-baremetal
 
 # Support airgapped build environment
 build:airgapped_env --//rules:build_env='airgapped'

constraints/extension/BUILD.bazel

diff --git a/constraints/extension/BUILD.bazel b/constraints/extension/BUILD.bazel
index f92a5c9..63ba63b 100644
--- a/constraints/extension/BUILD.bazel
+++ b/constraints/extension/BUILD.bazel
@@ -6,3 +6,7 @@
     name = "cheri",
     constraint_setting = ":extension",
 )
+constraint_value(
+    name = "cheri-baremetal",
+    constraint_setting = ":extension",
+)

platforms/cheri/riscv32/BUILD.bazel

diff --git a/platforms/cheri/riscv32/BUILD.bazel b/platforms/cheri/riscv32/BUILD.bazel
index 0a72252..14ca641 100644
--- a/platforms/cheri/riscv32/BUILD.bazel
+++ b/platforms/cheri/riscv32/BUILD.bazel
@@ -7,3 +7,10 @@
         "//constraints/extension:cheri",
     ],
 )
+platform(
+    name = "cheriot-baremetal",
+    constraint_values = [
+        "@platforms//cpu:riscv32",
+        "//constraints/extension:cheri-baremetal",
+    ],
+)

platforms/cheri/riscv32/devices.bzl

diff --git a/platforms/cheri/riscv32/devices.bzl b/platforms/cheri/riscv32/devices.bzl
index a0670c2..31a928f 100644
--- a/platforms/cheri/riscv32/devices.bzl
+++ b/platforms/cheri/riscv32/devices.bzl
@@ -25,4 +25,27 @@
             ),
         },
     ),
+    device_config(
+        name = "cheriot-baremetal",
+        architecture = "rv32imcxcheri",
+        feature_set = "//platforms/cheri/riscv32/features:rv32imcxcheri",
+        constraints = [
+            "@platforms//cpu:riscv32",
+            "//constraints/extension:cheri-baremetal",
+        ],
+        substitutions = {
+            "ARCHITECTURE": "rv32imcxcheri",
+            "ABI": "cheriot-baremetal",
+            "CMODEL": "medany",
+            "ENDIAN": "little",
+            "TARGET": "riscv32-unknown-elf",
+            "[STACK_PROTECTOR]": "",
+            "[SYSTEM_LIBRARY_PATHS]": listify_flags(
+                "-L{}",
+                [
+                    "external/cheriot-llvm/baremetal/baremetal-riscv32-purecap/riscv32-unknown-elf/lib/",
+                ],
+            ),
+        },
+    ),
 ]