gccrs: Add new compiler flag for parsing & compiling C-style string literals

gcc/rust/ChangeLog:
	* lang.opt: Add new -frust-c-style-string-literals option.
	* parse/rust-parse.h: Import options.h for reading flag_c_style_string_literals.
	* parse/rust-parse-impl-expr.hxx (Parser<ManagedTokenSource>::parse_literal_expr):
	Abort parsing C-style string literals if flag_c_style_string_literals is not set.
	(Parser<ManagedTokenSource>::null_denotation_not_path): Ditto.

gcc/testsuite/ChangeLog:
	* rust/execute/torture/c_string.rs: Set -frust-c-style-string-literals.
	* rust/compile/c_string_null_byte_check.rs: Set -frust-c-style-string-literals.

Signed-off-by: Yap Zhi Heng <yapzhhg@gmail.com>