Mark ld.so internel string functions hidden

Since ld.so internel string functions are only used internally in
ld.so, they can be made hidden.

	[BZ #19122]
	* include/string.h [IS_IN (rtld)] (__stpcpy): Add
	attribute_hidden.
	[IS_IN (rtld)] (__strdup): Likewise.
	[IS_IN (rtld)] (__strerror_r): Likewise.
	[IS_IN (rtld)] (__strsep_g): Likewise.
	[IS_IN (rtld)] (memchr): Likewise.
	[IS_IN (rtld)] (memcmp): Likewise.
	[IS_IN (rtld)] (memcpy): Likewise.
	[IS_IN (rtld)] (memmove): Likewise.
	[IS_IN (rtld)] (memset): Likewise.
	[IS_IN (rtld)] (rawmemchr): Likewise.
	[IS_IN (rtld)] (stpcpy): Likewise.
	[IS_IN (rtld)] (strchr): Likewise.
	[IS_IN (rtld)] (strcmp): Likewise.
	[IS_IN (rtld)] (strlen): Likewise.
	[IS_IN (rtld)] (strnlen): Likewise.
	[IS_IN (rtld)] (strsep): Likewise.