cmake: llvm: Make the setup for host installed clang easier

The logic is practically intact and is the following:

1. Use any host installed llvm/clang in the path in case
   ZEPHYR_TOOLCHAIN_VARIANT=llvm is requested alone.
2. This can be further restricted with TOOLCHAIN_HOME.
3. And can be further overridden with CLANG_ROOT_DIR,
   like previously.

So, only the unconditional restriction to /usr is lifted.

Together with fixing the unconditional set of TOOLCHAIN_HOME
by host tools for non-toolchain needs, this makes the logic
more flexible.

Now, after the logic is controllable by TOOLCHAIN_HOME, 3)
might be an extra, but is left intact for backward compatibility.

Signed-off-by: Oleg Zhurakivskyy <[email protected]>

Author: ozhuraki

cmake/toolchain/llvm/generic.cmake

@@ -1,9 +1,8 @@
 # SPDX-License-Identifier: Apache-2.0
 
-set(CLANG_ROOT $ENV{CLANG_ROOT_DIR})
-set_ifndef(CLANG_ROOT /usr)
-
-set(TOOLCHAIN_HOME ${CLANG_ROOT}/bin/)
+if(DEFINED $ENV{CLANG_ROOT_DIR})
+  set(TOOLCHAIN_HOME ${CLANG_ROOT}/bin/)
+endif()
 
 set(COMPILER clang)
 set(LINKER ld) # TODO: Use lld eventually rather than GNU ld
@@ -18,3 +17,7 @@ endif()
 set(CMAKE_C_COMPILER_TARGET   ${triple})
 set(CMAKE_ASM_COMPILER_TARGET ${triple})
 set(CMAKE_CXX_COMPILER_TARGET ${triple})
+
+if("${ARCH}" STREQUAL "posix")
+  set(TOOLCHAIN_HAS_NEWLIB OFF CACHE BOOL "True if toolchain supports newlib")
+endif()