x86-64: Mark internal symbols with attribute_hidden [BZ #18822]

Since __syscall_clock_gettime and __start_context are internal symbols
for Linux/x86-64, mark them with attribute_hidden.

	[BZ #18822]
	* sysdeps/unix/sysv/linux/x86_64/init-first.c
	(__syscall_clock_gettime): Add attribute_hidden.
	* sysdeps/unix/sysv/linux/x86_64/makecontext.c
	(__start_context): Likewise.