gccrs: Add option_env! support

gccrs: Add option_env! support

gcc/rust/ChangeLog:
	* expand/rust-macro-builtins-utility.cc: Add macro expansion for
	option_env with eager expansion
	* expand/rust-macro-builtins.cc: Add option_env to builtin list
	* expand/rust-macro-builtins.h: Add option_env handler to header
	file
	* resolve/rust-late-name-resolver-2.0.cc: Prevent NR2.0 from
	recursing into lang-item segments

gcc/testsuite/ChangeLog:
	* rust/compile/macros/builtin/option_env1.rs: Add success case for option_env
	* rust/compile/macros/builtin/option_env2.rs: Add failure case for option_env
	* rust/execute/torture/builtin_macro_option_env.rs: Add
	execution case for option_env