riscv: kexec: add kexec_file_load() support

This patch set implements kexec_file_load() for RISC-V, which is
currently only allowed on rv64 due to some minor build issues on 32-bit
platforms in the generic code.  This allows users to kexec() using an FD
as opposed to a buffer.

Link: https://lore.kernel.org/all/20220408100914.150110-1-lizhengyu3@huawei.com/

* palmer/riscv-kexec_file:
  RISC-V: Load purgatory in kexec_file
  RISC-V: Add purgatory
  RISC-V: Support for kexec_file on panic
  RISC-V: Add kexec_file support
  RISC-V: use memcpy for kexec_file mode
  kexec_file: Fix kexec_file.c build error for riscv platform