lib: c: combine libc and ulibc

libc is now made up of several independent components, each of which is individually compiled into a static library.
they are then all combined into a single shared library.

lib/libc/include/string.h

@@ -3,6 +3,9 @@
 
 #include <stddef.h>
 
+extern const char *strerror(int errnum);
+extern const char *strerror_code(int errnum);
+
 extern size_t strlen(const char *s);
 
 extern int strcmp(const char *s1, const char *s2);