Rename remaining members of buildsym_compunit to start with "m_"

Rename remaining members of buildsym_compunit to start with "m_"

This renames all the remaining members of buildsym_compunit to start
with "m_" to follow the general naming convention.

gdb/ChangeLog
2018-07-20  Keith Seitz  <keiths@redhat.com>

	* buildsym.h (struct buildsym_compunit) <m_objfile, m_subfiles,
	m_main_subfile, m_comp_dir, m_producer, m_debugformat,
	m_compunit_symtab, m_language>: Add "m_" prefix.
	Update all uses.
	* buildsym.c: Update all uses.