gccrs: Fix float lexing and tuple index disambiguation

When a float has a floating point but no value after it, a zero was added
this lead to errors when trying to disambiguate a float into a tuple
index.

gcc/rust/ChangeLog:

	* lex/rust-lex.cc (Lexer::parse_decimal_int_or_float): Remove
	additional zero after empty floating point.
	* parse/rust-parse-impl.h (Parser::left_denotation): Handle float with
	empty floating point.

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