riscv/purgatory: 4B align purgatory_start

riscv/purgatory: 4B align purgatory_start

When a crashkernel is launched on RISC-V, the entry to purgatory is
done by trapping via the stvec CSR. From riscv_kexec_norelocate():

  |  ...
  |  /*
  |   * Switch to physical addressing
  |   * This will also trigger a jump to CSR_STVEC
  |   * which in this case is the address of the new
  |   * kernel.
  |   */
  |  csrw    CSR_STVEC, a2
  |  csrw    CSR_SATP, zero

stvec requires that the address is 4B aligned, which was not the case,
e.g.:

  | Loaded purgatory at 0xffffc000
  | kexec_file: kexec_file_load: type:1, start:0xffffd232 head:0x4 flags:0x6

The address 0xffffd232 not 4B aligned.

Correct by adding proper function alignment.

With this change, crashkernels loaded with kexec-file will be able to
properly enter the purgatory.

Fixes: 736e30af583fb ("RISC-V: Add purgatory")
Signed-off-by: Björn Töpel <bjorn@rivosinc.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20250328085313.1193815-1-bjorn@kernel.org
Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>