cmake: Add LLVM toolchain support

This commit adds the support file for the `zephyr-llvm` toolchain
variant.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>

cmake/zephyr/Kconfig

@@ -4,7 +4,8 @@ config TOOLCHAIN_ZEPHYR_0_17
 	def_bool y
 	# FIXME: Newlib-nano is disabled for Xtensa targets due to the memset
         #        bug causing crashes (see the GitHub issue #660).
-	select HAS_NEWLIB_LIBC_NANO if !XTENSA
+	select HAS_NEWLIB_LIBC_NANO if !XTENSA \
+				    && "$(ZEPHYR_TOOLCHAIN_VARIANT)" != "zephyr-llvm"
 
 config TOOLCHAIN_ZEPHYR_SUPPORTS_THREAD_LOCAL_STORAGE
 	def_bool y
@@ -16,6 +17,7 @@ config TOOLCHAIN_ZEPHYR_SUPPORTS_GNU_EXTENSIONS
 
 config PICOLIBC_SUPPORTED
 	def_bool y
-	depends on "$(ZEPHYR_TOOLCHAIN_VARIANT)" = "zephyr-gnu"
+	depends on "$(ZEPHYR_TOOLCHAIN_VARIANT)" = "zephyr-gnu" \
+		|| "$(ZEPHYR_TOOLCHAIN_VARIANT)" = "zephyr-llvm"
 	help
 	  Zephyr SDK >=0.16 always supports Picolibc for C and C++ development.

cmake/zephyr/llvm/clang_compiler_rt.cfg

@@ -0,0 +1,4 @@
+# Copyright (c) 2023 The ChromiumOS Authors
+# SPDX-License-Identifier: Apache-2.0
+
+--rtlib=compiler-rt

cmake/zephyr/llvm/generic.cmake

@@ -0,0 +1,11 @@
+# SPDX-License-Identifier: Apache-2.0
+
+set(LLVM_TOOLCHAIN_PATH ${ZEPHYR_SDK_INSTALL_DIR}/llvm)
+
+set(TOOLCHAIN_HOME ${ZEPHYR_SDK_INSTALL_DIR}/llvm/bin)
+
+set(COMPILER clang)
+set(LINKER lld)
+set(BINTOOLS llvm)
+
+set(TOOLCHAIN_HAS_PICOLIBC ON CACHE BOOL "True if toolchain supports picolibc")

cmake/zephyr/llvm/target.cmake

@@ -0,0 +1,38 @@
+# SPDX-License-Identifier: Apache-2.0
+
+if("${ARCH}" STREQUAL "arm")
+  if(DEFINED CONFIG_ARMV8_M_MAINLINE)
+    # ARMv8-M mainline is ARMv7-M with additional features from ARMv8-M.
+    set(triple armv8m.main-none-eabi)
+  elseif(DEFINED CONFIG_ARMV8_M_BASELINE)
+    # ARMv8-M baseline is ARMv6-M with additional features from ARMv8-M.
+    set(triple armv8m.base-none-eabi)
+  elseif(DEFINED CONFIG_ARMV7_M_ARMV8_M_MAINLINE)
+    # ARMV7_M_ARMV8_M_MAINLINE means that ARMv7-M or backward compatible ARMv8-M
+    # processor is used.
+    set(triple armv7m-none-eabi)
+  elseif(DEFINED CONFIG_ARMV6_M_ARMV8_M_BASELINE)
+    # ARMV6_M_ARMV8_M_BASELINE means that ARMv6-M or ARMv8-M supporting the
+    # Baseline implementation processor is used.
+    set(triple armv6m-none-eabi)
+  else()
+    # Default ARM target supported by all processors.
+    set(triple arm-none-eabi)
+  endif()
+elseif("${ARCH}" STREQUAL "arm64")
+  set(triple aarch64-none-elf)
+# TODO: Add RISC-V target support.
+endif()
+
+if(DEFINED triple)
+  set(CMAKE_C_COMPILER_TARGET   ${triple})
+  set(CMAKE_ASM_COMPILER_TARGET ${triple})
+  set(CMAKE_CXX_COMPILER_TARGET ${triple})
+
+  unset(triple)
+endif()
+
+list(APPEND TOOLCHAIN_C_FLAGS --config
+	${ZEPHYR_SDK_INSTALL_DIR}/cmake/zephyr/llvm/clang_compiler_rt.cfg)
+list(APPEND TOOLCHAIN_LD_FLAGS --config
+	${ZEPHYR_SDK_INSTALL_DIR}/cmake/zephyr/llvm/clang_compiler_rt.cfg)