riscv: use local label names instead of global ones in assembly

riscv: use local label names instead of global ones in assembly

Local labels should be prefix by '.L' or they'll be exported in the
symbol table. Additionally, this messes up the backtrace by displaying
an incorrect symbol:

  ...
  [   12.751810] [<ffffffff80441628>] _copy_from_user+0x28/0xc2
  [   12.752035] [<ffffffff800152ca>] handle_misaligned_load+0x1ca/0x2fc
  [   12.752310] [<ffffffff80a033e8>] do_trap_load_misaligned+0x24/0xee
  [   12.752596] [<ffffffff80a0dcae>] _new_vmalloc_restore_context_a0+0xc2/0xce

After:
  ...
  [   10.243916] [<ffffffff804415e4>] _copy_from_user+0x28/0xc2
  [   10.244026] [<ffffffff800152ca>] handle_misaligned_load+0x1ca/0x2fc
  [   10.244150] [<ffffffff80a033a0>] do_trap_load_misaligned+0x24/0xee
  [   10.244268] [<ffffffff80a0dc66>] handle_exception+0x146/0x152

Signed-off-by: Clément Léger <cleger@rivosinc.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Fixes: 503638e0babf3 ("riscv: Stop emitting preventive sfence.vma for new vmalloc mappings")
Link: https://lore.kernel.org/r/20250103141814.508865-1-cleger@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>