fe310: Support the LLVM toolchain (i.e. compilation with clang)

This requires -nostartfiles to be only passed to the linker, not the
compiler, as it is a linker flag and passing it to the compiler causes a
clang warning to be emitted.

Additionally, clang does not seem to support `-mcmodel=medlow` and
`-msmall-data-limit=8` but these options do not seem strictly necessary
to me anyhow thus they are deactivated conditionally when using clang.

cpu/riscv_common/Makefile.include

@@ -1,6 +1,8 @@
 CFLAGS += -Wno-pedantic
 INCLUDES += -I$(RIOTCPU)/riscv_common/include
 
+TOOLCHAINS_SUPPORTED = gnu llvm
+
 # All variables must be defined in the CPU configuration when using the common
 # `ldscripts/riscv.ld`
 ifneq (,$(ROM_START_ADDR)$(RAM_START_ADDR)$(ROM_LEN)$(RAM_LEN))

makefiles/arch/riscv.inc.mk

@@ -30,8 +30,15 @@ TARGET_ARCH_RISCV ?= \
 TARGET_ARCH ?= $(TARGET_ARCH_RISCV)
 
 # define build specific options
-CFLAGS_CPU   = -march=rv32imac -mabi=ilp32 -mcmodel=medlow -msmall-data-limit=8
-CFLAGS_LINK  = -nostartfiles -ffunction-sections -fdata-sections
+CFLAGS_CPU   = -march=rv32imac -mabi=ilp32
+ifeq ($(TOOLCHAIN),llvm)
+  # Always use riscv32-none-elf as target triple for clang, as some
+  # autodetected gcc target triples are incompatible with clang
+  TARGET_ARCH_LLVM := riscv32-none-elf
+else
+  CFLAGS_CPU += -mcmodel=medlow -msmall-data-limit=8
+endif
+CFLAGS_LINK  = -ffunction-sections -fdata-sections
 CFLAGS_DBG  ?= -g3
 CFLAGS_OPT  ?= -Os
 
@@ -42,4 +49,4 @@ LINKFLAGS += -T$(LINKER_SCRIPT)
 CFLAGS += $(CFLAGS_CPU) $(CFLAGS_DBG) $(CFLAGS_OPT) $(CFLAGS_LINK)
 ASFLAGS += $(CFLAGS_CPU) $(CFLAGS_DBG)
 # export linker flags
-LINKFLAGS += $(CFLAGS_CPU) $(CFLAGS_LINK) $(CFLAGS_DBG) $(CFLAGS_OPT) -Wl,--gc-sections -static -lgcc
+LINKFLAGS += $(CFLAGS_CPU) $(CFLAGS_LINK) $(CFLAGS_DBG) $(CFLAGS_OPT) -nostartfiles -Wl,--gc-sections -static -lgcc