gccrs: Add new keyword utility class

gccrs: Add new keyword utility class

Much like attributes values, keywords are known beforehand and never
change. Instead of relying on handcrafted string we could centralize
everything in one place. We may require to check whether a word is a
keyword, which can now be done easily thanks to the keyword set.

gcc/rust/ChangeLog:

	* Make-lang.in: Add rust-keyword-values.cc to the list.
	* util/rust-keyword-values.cc: New file.
	* util/rust-keyword-values.h: New file.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>