libc/string: prevent libc in the kernel/userspace optionally

Add the `LIBC_PREVENT_STRING_KERNEL` and `LIBC_PREVENT_STRING_USER`
that are meant to be selected by the chip if no libc implementation
is going to be built. If selected, neither NuttX's software version
of the libc nor any architecture-specific implementation will be
built in the kernel or in the userspace, respectively. In this
case, the linker may provide a ROM-defined version of the libc
functions instead.

libs/libc/machine/risc-v/gnu/arch_memcpy.S

@@ -18,6 +18,14 @@
  *
  ****************************************************************************/
 
+/************************************************************************************
+ * Included Files
+ ************************************************************************************/
+
+#include "libc.h"
+
+#ifdef LIBC_BUILD_STRING
+
 /************************************************************************************
  * Public Symbols
  ************************************************************************************/
@@ -128,3 +136,5 @@ memcpy:
 	bltu		a1, a3, 5b
 6:
 	ret
+
+#endif

libs/libc/machine/risc-v/gnu/arch_memset.S

@@ -13,6 +13,10 @@
  *
  ****************************************************************************/
 
+#include "libc.h"
+
+#ifdef LIBC_BUILD_STRING
+
 .text
 .global memset
 .type	memset, @function
@@ -104,3 +108,5 @@ memset:
 	bleu a2, t1, .Ltiny
 	j .Laligned
 	.size	memset, .-memset
+
+#endif

libs/libc/machine/risc-v/gnu/arch_strcmp.S

@@ -13,6 +13,10 @@
  *
  ****************************************************************************/
 
+#include "libc.h"
+
+#ifdef LIBC_BUILD_STRING
+
 #include "asm.h"
 
 .text
@@ -185,3 +189,5 @@ strcmp:
 mask:
 .dword 0x7f7f7f7f7f7f7f7f
 #endif
+
+#endif