xml_fetch_content_from_file: Read in whole file in one go

xml_fetch_content_from_file: Read in whole file in one go

There doesn't seem to be a good reason we're reading the file one
chunk at a time.

gdb/ChangeLog:
2017-10-19  Pedro Alves  <palves@redhat.com>

	* xml-support.c (xml_fetch_content_from_file): Don't read in
	chunks.  Instead use fseek to determine the file's size, and read
	it in one go.