gccrs: Change keyword set to a map

gccrs: Change keyword set to a map

Some part of the code requires the token id behind a given keyword, a map
keep the "set" aspect whilst providing this additional feature.

gcc/rust/ChangeLog:

	* lex/rust-lex.cc (RS_TOKEN): Remove local map.
	(RS_TOKEN_KEYWORD): Likewise.
	(Lexer::classify_keyword): Change call to utils.
	* util/rust-keyword-values.cc (get_keywords): Add init function.
	(RS_TOKEN_KEYWORD): Call to X macro.
	* util/rust-keyword-values.h: Change from set to a map.

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