clang: Don't specify FP16 format

Support for 16 bit floats is enabled by default in Clang [1].
The ARM alternative format is not supported, so __fp16 always uses
IEEE 754-2008 format [2].

[1] https://github.com/llvm/llvm-project/blob/main/llvm/lib/Target/ARM/ARMAsmPrinter.cpp#L750-L755
[2] https://clang.llvm.org/docs/LanguageExtensions.html#half-precision-floating-point

Fixes: #55562

Signed-off-by: Patryk Duda <pdk@semihalf.com>

arch/arm/core/aarch32/Kconfig

@@ -312,6 +312,8 @@ config FP16_ALT
 	  so that this format can represent normalized values in the range of
 	  2^(-14) to 131008.
 
+	  Please note that Clang doesn't support the ARM alternative format.
+
 endchoice
 
 rsource "cortex_m/Kconfig"

cmake/compiler/clang/target.cmake

@@ -29,7 +29,7 @@ if(NOT "${ARCH}" STREQUAL "posix")
       -fshort-enums
       )
 
-    include(${ZEPHYR_BASE}/cmake/compiler/gcc/target_arm.cmake)
+    include(${ZEPHYR_BASE}/cmake/compiler/clang/target_arm.cmake)
   endif()
 
   foreach(file_name include/stddef.h)

cmake/compiler/clang/target_arm.cmake

@@ -0,0 +1,38 @@
+# SPDX-License-Identifier: Apache-2.0
+
+set(ARM_C_FLAGS)
+
+list(APPEND ARM_C_FLAGS   -mcpu=${GCC_M_CPU})
+
+if(CONFIG_COMPILER_ISA_THUMB2)
+  list(APPEND ARM_C_FLAGS   -mthumb)
+endif()
+
+list(APPEND ARM_C_FLAGS -mabi=aapcs)
+
+if(CONFIG_FPU)
+  list(APPEND ARM_C_FLAGS   -mfpu=${GCC_M_FPU})
+
+  if(CONFIG_DCLS AND NOT CONFIG_FP_HARDABI)
+    # If the processor is equipped with VFP and configured in DCLS topology,
+    # the FP "hard" ABI must be used in order to facilitate the FP register
+    # initialisation and synchronisation.
+    set(FORCE_FP_HARDABI TRUE)
+  endif()
+
+  if    (CONFIG_FP_HARDABI OR FORCE_FP_HARDABI)
+    list(APPEND ARM_C_FLAGS   -mfloat-abi=hard)
+  elseif(CONFIG_FP_SOFTABI)
+    list(APPEND ARM_C_FLAGS   -mfloat-abi=softfp)
+  endif()
+endif()
+
+if(CONFIG_FP16)
+  # Clang only supports IEEE 754-2008 format for __fp16. It's enabled by
+  # default, so no need to do anything when CONFIG_FP16_IEEE is selected.
+  if(CONFIG_FP16_ALT)
+    message(FATAL_ERROR "Clang doesn't support ARM alternative format for FP16")
+  endif()
+endif()
+list(APPEND TOOLCHAIN_C_FLAGS ${ARM_C_FLAGS})
+list(APPEND TOOLCHAIN_LD_FLAGS NO_SPLIT ${ARM_C_FLAGS})