riscv: simplify the stack pointer setup in head.S

We don't need THREAD_SIZE in asm-offsets.c as we can just calculate
the value of init_thread_union + THREAD_SIZE using cpp, just like
we do a few lines above.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Atish Patra <atish.patra@wdc.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>