riscv: ensure arguments to __ASM_STR() are expanded first

Right now this does not cause any issues, but I had a local change where
missing expansion resulted in compilation errors when reading CSRs.

arch/riscv/include/arch/riscv.h

@@ -134,7 +134,8 @@
 #define RISCV_EXCEPTION_STORE_PAGE_FAULT    15
 
 #ifndef ASSEMBLY
-#define __ASM_STR(x)    #x
+#define ___ASM_STR(x)    #x
+#define __ASM_STR(x)    ___ASM_STR(x)
 
 __BEGIN_CDECLS