riscv: Add a Zalrsc-only alternative for synchronization in start.S

Add an alternative implementation that use Zalrsc extension only for
HART lottery and SMP locking to support SMP on cores without "Zaamo"
extension available. The Zaamo implementation is still prioritized if
both of them are available, since it takes fewer instructions.

Signed-off-by: Yao Zi <ziyao@disroot.org>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>

arch/riscv/cpu/start.S

@@ -151,8 +151,15 @@ call_harts_early_init:
 	 */
 	la	t0, hart_lottery
 	li	t1, 1
+#if CONFIG_IS_ENABLED(RISCV_ISA_ZAAMO)
 	amoswap.w s2, t1, 0(t0)
 	bnez	s2, wait_for_gd_init
+#else
+	lr.w	s2, (t0)
+	bnez	s2, wait_for_gd_init
+	sc.w	s2, t1, (t0)
+	bnez	s2, wait_for_gd_init
+#endif
 #else
 	/*
 	 * FIXME: gp is set before it is initialized. If an XIP U-Boot ever
@@ -177,7 +184,12 @@ call_harts_early_init:
 #if !CONFIG_IS_ENABLED(XIP)
 #ifdef CONFIG_AVAILABLE_HARTS
 	la	t0, available_harts_lock
+#if CONFIG_IS_ENABLED(RISCV_ISA_ZAAMO)
 	amoswap.w.rl zero, zero, 0(t0)
+#else
+	fence	rw, w
+	sw	zero, 0(t0)
+#endif
 #endif
 
 wait_for_gd_init:
@@ -190,7 +202,14 @@ wait_for_gd_init:
 #ifdef CONFIG_AVAILABLE_HARTS
 	la	t0, available_harts_lock
 	li	t1, 1
-1:	amoswap.w.aq t1, t1, 0(t0)
+1:
+#if CONFIG_IS_ENABLED(RISCV_ISA_ZAAMO)
+	amoswap.w.aq t1, t1, 0(t0)
+#else
+	lr.w.aq	t1, 0(t0)
+	bnez	t1, 1b
+	sc.w.rl t1, t1, 0(t0)
+#endif
 	bnez	t1, 1b
 
 	/* register available harts in the available_harts mask */
@@ -200,7 +219,12 @@ wait_for_gd_init:
 	or	t2, t2, t1
 	SREG	t2, GD_AVAILABLE_HARTS(gp)
 
+#if CONFIG_IS_ENABLED(RISCV_ISA_ZAAMO)
 	amoswap.w.rl zero, zero, 0(t0)
+#else
+	fence	rw, w
+	sw	zero, 0(t0)
+#endif
 #endif
 
 	/*