Fix setting incorrect sp if single-hart

[ivq]

The PR #119 introduced a bug that if feature single-hart is enabled, stack pointer is
not set properly. That is:

# near _abs_start
# ...
20000042:	4e81                	li	t4,0
20000044:	4f01                	li	t5,0
20000046:	4f81                	li	t6,0

20000048 <.Lpcrel_hi0>:
20000048:	60000197          	auipc	gp,0x60000
2000004c:	7b818193          	addi	gp,gp,1976 # 80000800 <__global_pointer$>

20000050 <.Lpcrel_hi1>:
20000050:	60008117          	auipc	sp,0x60008
20000054:	fb010113          	addi	sp,sp,-80 # 80008000 <_sstack>
20000058:	000012b7          	lui	t0,0x1
2000005c:	80028293          	addi	t0,t0,-2048 # 800 <_hart_stack_size>
20000060:	40510133          	sub	sp,sp,t0 # BUG here: sp is set to _sstack(equals to _stack_start) - _hart_stack_size
20000064:	840a                	mv	s0,sp
20000066:	0040006f          	j	2000006a <_start_rust>

The stack pointer should be set to _stack_start in this case. Also, the operations on _hart_stack_size should be removed.

After the fix:

# near _abs_start
# ...
20000042:	4e81                	li	t4,0
20000044:	4f01                	li	t5,0
20000046:	4f81                	li	t6,0

20000048 <.Lpcrel_hi0>:
20000048:	60000197          	auipc	gp,0x60000
2000004c:	7b818193          	addi	gp,gp,1976 # 80000800 <__global_pointer$>

20000050 <.Lpcrel_hi1>:
20000050:	60008117          	auipc	sp,0x60008
20000054:	fb010113          	addi	sp,sp,-80 # 80008000 <_sstack>
20000058:	840a                	mv	s0,sp
2000005a:	0040006f          	j	2000005e <_start_rust>

Relevant document is also updated.

[romancardenas]

LGTM