Remove cleanups from coff-pe-read.c

Remove cleanups from coff-pe-read.c

This removes cleanups from coff-pe-read.c, using std::string,
std::vector, and gdb::def_vector.

Tested by the buildbot, though I'm not sure these code paths are
exercised there.

ChangeLog
2018-05-25  Tom Tromey  <tom@tromey.com>

	* coff-pe-read.c (struct read_pe_section_data) <section_name>: Now
	a std::string.
	(get_pe_section_index, add_pe_exported_sym): Update.
	(read_pe_exported_syms): Use gdb::def_vector.