x86-64/memset: Mark the debugger symbol as hidden

When MEMSET_SYMBOL (__memset, erms) is provided for debugger, mark it
as hidden so that it will be local to the library.

	* sysdeps/x86_64/multiarch/memset-vec-unaligned-erms.S
	(MEMSET_SYMBOL (__memset, erms)): Mark the debugger symbol as
	hidden.