Zero-initialize linux note sections

Zero-initialize linux note sections

This patches changes linux-tdep.c so that the buffer used to write
note sections when generating a core file is zero-initialized.  This
way, bytes that are not collected won't contain random
data (e.g. padding bytes).

gdb/ChangeLog:
2018-10-26  Pedro Franco de Carvalho  <pedromfc@linux.ibm.com>

	* linux-tdep.c (linux_collect_regset_section_cb): Use
	std::vector<gdb_byte> instead of char * and malloc for buf.
	Remove xfree.