riscv: add platform-specific double word shifts for riscv32

Add riscv32-specific '__ashldi3()', '__ashrdi3()', and '__lshrdi3()'.
Initially it was intended to fix the following link error observed

riscv: add platform-specific double word shifts for riscv32

Add riscv32-specific '__ashldi3()', '__ashrdi3()', and '__lshrdi3()'.
Initially it was intended to fix the following link error observed when
building EFI-enabled kernel with CONFIG_EFI_STUB=y and
CONFIG_EFI_GENERIC_STUB=y:

riscv32-linux-gnu-ld: ./drivers/firmware/efi/libstub/lib-cmdline.stub.o: in function `__efistub_.L49':
__efistub_cmdline.c:(.init.text+0x1f2): undefined reference to `__efistub___ashldi3'
riscv32-linux-gnu-ld: __efistub_cmdline.c:(.init.text+0x202): undefined reference to `__efistub___lshrdi3'

Reported at [1] trying to build
https://patchew.org/linux/20260212164413.889625-1-dmantipov@yandex.ru,
tested with 'qemu-system-riscv32 -M virt' only.

Link: https://lore.kernel.org/20260519172259.908980-7-dmantipov@yandex.ru
Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202603041925.KLKqpK6N-lkp@intel.com [1]
Suggested-by: Ard Biesheuvel <ardb@kernel.org>
Tested-by: Charlie Jenkins <thecharlesjenkins@gmail.com>
Assisted-by: Gemini:gemini-3.1-pro-preview sashiko
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: Alexandre Ghiti <alex@ghiti.fr>
Cc: Andriy Shevchenko <andriy.shevchenko@intel.com>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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