Minimal executable, cleaned up from vendor code

- Removed extraneous and/or oversized(?) align from linker script
- Handle init_array properly
- Remove legacy init code paths (.ctors, .init)
- Don't do nonstandard interrupt mode handling by default

minimal-payload/Makefile

@@ -0,0 +1,25 @@
+.PHONY: all clean
+
+RV_ARCH = rv32imacxw
+
+CC = riscv-none-elf-gcc
+OBJDUMP = riscv-none-elf-objdump
+CFLAGS = -ggdb3 -Os -march=$(RV_ARCH) -ffunction-sections -fdata-sections
+LDFLAGS = -ggdb3 -march=$(RV_ARCH) -Wl,--gc-sections --specs=nosys.specs
+
+all: main.elf
+
+main.elf: startup.o main.o
+
+%.elf: linker.lds
+	$(CC) -Xlinker -Map=$(@:.elf=.map) -Wl,--script=linker.lds -nostartfiles -o $@ $(filter-out linker.lds,$+)
+	$(OBJDUMP) -xdsS $@ >$(@:.elf=.dump)
+
+%.o: %.S
+	$(CC) $(CFLAGS) -c -o $@ $<
+
+%.o: %.c
+	$(CC) $(CFLAGS) -c -o $@ $<
+
+clean:
+	rm -f *.o *.elf *.map *.dump