gdb: Add new parser rule for structure field names

gdb: Add new parser rule for structure field names

Introduces a new rule in c-exp.y for matching structure field names.

This is a restructure in preparation for the next commit, this commit
shouldn't result in any user visible changes.

gdb/ChangeLog:

	* c-exp.y (field_name): New %token, and new rule.
	(exp): Replace uses of 'name' with 'field_name' where appropriate.