[arch][riscv] C++ header guard the core riscv header

arch/riscv/include/arch/riscv.h

@@ -109,6 +109,8 @@
 #ifndef ASSEMBLY
 #define __ASM_STR(x)    #x
 
+__BEGIN_CDECLS
+
 #define riscv_csr_clear(csr, bits) \
 ({ \
     ulong __val = bits; \
@@ -238,4 +240,6 @@ static inline uint64_t riscv_get_time(void) {
 # endif
 #endif
 
+__END_CDECLS
+
 #endif /* ASSEMBLY */