gccrs: lex: Emit E0768 for empty non-decimal literals

Previously, the lexer evaluated empty non-decimal literals (like 0x, 0b, 0o) as 0.
Now, it emits error E0768 when there are no valid digits.

gcc/rust/ChangeLog:

	* lex/rust-lex.cc (Lexer::parse_non_decimal_int_literal): Emit E0768.

gcc/testsuite/ChangeLog:

	* rust/compile/empty-non-decimal.rs: New test.

Signed-off-by: Enes Cevik <nsvke@proton.me>