riscv: Add initial support for 32bit boards

* Adding separate targets for 32bit and 64bit qemu
* Using the riscv64 toolchain for 32bit builds requires setting -m elf32lriscv
* rv32/rv64 is currently configured with ARCH_RISCV_RV32/RV64 and not per stage.
  This should probably be changed later.

TEST=Boots to "Payload not loaded." on 32bit qemu using the following commands:

util/riscv/make-spike-elf.sh build/coreboot.rom build/coreboot.elf
qemu-system-riscv32 -M virt -m 1024M -nographic -kernel build/coreboot.elf

Change-Id: I35e59b459d1770df10b51fe9e77dcc474d7c75a0
Signed-off-by: Philipp Hug <philipp@hug.cx>
Reviewed-on: https://review.coreboot.org/c/31253
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: ron minnich <rminnich@gmail.com>

src/lib/libgcc.c

@@ -20,7 +20,8 @@
  * <lib.h> in case GCC does not have an assembly version for this arch.
  */
 
-#if !IS_ENABLED(CONFIG_ARCH_X86) /* work around lack of --gc-sections on x86 */
+#if !IS_ENABLED(CONFIG_ARCH_X86)	       /* work around lack of --gc-sections on x86 */ \
+	&& !IS_ENABLED(CONFIG_ARCH_RISCV_RV32) /* defined in rv32 libgcc.a */
 int __clzsi2(u32 a);
 int __clzsi2(u32 a)
 {