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>

arch/riscv/kernel/asm-offsets.c

@@ -312,9 +312,6 @@ void asm_offsets(void)
 		- offsetof(struct task_struct, thread.fstate.f[0])
 	);
 
-	/* The assembler needs access to THREAD_SIZE as well. */
-	DEFINE(ASM_THREAD_SIZE, THREAD_SIZE);
-
 	/*
 	 * We allocate a pt_regs on the stack when entering the kernel.  This
 	 * ensures the alignment is sane.

arch/riscv/kernel/head.S

@@ -69,10 +69,7 @@ clear_bss_done:
 	/* Restore C environment */
 	la tp, init_task
 	sw zero, TASK_TI_CPU(tp)
-
-	la sp, init_thread_union
-	li a0, ASM_THREAD_SIZE
-	add sp, sp, a0
+	la sp, init_thread_union + THREAD_SIZE
 
 	/* Start the kernel */
 	mv a0, s0