selftests/nolibc: allow specify extra arguments for qemu

The opensbi package from Ubuntu 20.04 only provides rv64 firmwares:

    $ dpkg -S opensbi | grep -E "fw_.*bin|fw_.*elf" | uniq
    opensbi: /usr/lib/riscv64-linux-gnu/opensbi/generic/fw_dynamic.bin
    opensbi: /usr/lib/riscv64-linux-gnu/opensbi/generic/fw_jump.bin
    opensbi: /usr/lib/riscv64-linux-gnu/opensbi/generic/fw_dynamic.elf
    opensbi: /usr/lib/riscv64-linux-gnu/opensbi/generic/fw_jump.elf

To run this nolibc test for rv32, users must build opensbi or download a
prebuilt one from qemu repository:

    https://gitlab.com/qemu-project/qemu/-/blob/master/pc-bios/opensbi-riscv32-generic-fw_dynamic.bin

And then use -bios to tell qemu use it to avoid such failure:

    $ qemu-system-riscv32 -display none -no-reboot -kernel /path/to/arch/riscv/boot/Image -serial stdio -M virt -append "console=ttyS0 panic=-1"
    qemu-system-riscv32: Unable to load the RISC-V firmware "opensbi-riscv32-generic-fw_dynamic.bin"

To run from makefile, QEMU_ARGS_EXTRA is added to allow pass extra
arguments like -bios:

    $ make run QEMU_ARGS_EXTRA="-bios /path/to/opensbi-riscv32-generic-fw_dynamic.bin" ...
Suggested-by: Thomas Weißschuh <linux@weissschuh.net>
Link: https://lore.kernel.org/linux-riscv/2ab94136-d341-4a26-964e-6d6c32e66c9b@t-8ch.de/Signed-off-by: Zhangjin Wu <falcon@tinylab.org>
Reviewed-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

tools/testing/selftests/nolibc/Makefile

@@ -64,7 +64,7 @@ QEMU_ARGS_mips       = -M malta -append "panic=-1 $(TEST:%=NOLIBC_TEST=%)"
 QEMU_ARGS_riscv      = -M virt -append "console=ttyS0 panic=-1 $(TEST:%=NOLIBC_TEST=%)"
 QEMU_ARGS_s390       = -M s390-ccw-virtio -m 1G -append "console=ttyS0 panic=-1 $(TEST:%=NOLIBC_TEST=%)"
 QEMU_ARGS_loongarch  = -M virt -append "console=ttyS0,115200 panic=-1 $(TEST:%=NOLIBC_TEST=%)"
-QEMU_ARGS            = $(QEMU_ARGS_$(ARCH))
+QEMU_ARGS            = $(QEMU_ARGS_$(ARCH)) $(QEMU_ARGS_EXTRA)
 
 # OUTPUT is only set when run from the main makefile, otherwise
 # it defaults to this nolibc directory.