riscv: vdso.lds.S: drop __alt_start and __alt_end symbols

These two symbols are not used, remove them.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Tested-by: Emil Renner Berthing <emil.renner.berthing@canonical.com>
Link: https://lore.kernel.org/r/20230912072015.2424-2-jszhang@kernel.org


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>