riscv: Add Zbb support for building U-Boot

This patch adds ISA string to the -march to generate zbb instructions
for U-Boot binaries, along with optimized string functions introduced
from Linux kernel.

Signed-off-by: Yu Chien Peter Lin <peterlin@andestech.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>
diff --git a/arch/riscv/include/asm/string.h b/arch/riscv/include/asm/string.h
index 7dee3e4..38ad85f 100644
--- a/arch/riscv/include/asm/string.h
+++ b/arch/riscv/include/asm/string.h
@@ -40,4 +40,22 @@
 #endif
 extern void *memset(void *, int, __kernel_size_t);
 
+#undef __HAVE_ARCH_STRLEN
+#if CONFIG_IS_ENABLED(USE_ARCH_STRLEN)
+#define __HAVE_ARCH_STRLEN
+#endif
+extern __kernel_size_t strlen(const char *);
+
+#undef __HAVE_ARCH_STRCMP
+#if CONFIG_IS_ENABLED(USE_ARCH_STRCMP)
+#define __HAVE_ARCH_STRCMP
+#endif
+extern int strcmp(const char *, const char *);
+
+#undef __HAVE_ARCH_STRNCMP
+#if CONFIG_IS_ENABLED(USE_ARCH_STRNCMP)
+#define __HAVE_ARCH_STRNCMP
+#endif
+extern int strncmp(const char *, const char *, size_t __kernel_size_t);
+
 #endif /* __ASM_RISCV_STRING_H */