gccrs: expand: Add parser for offset_of!() and builtin resolution.

gccrs: expand: Add parser for offset_of!() and builtin resolution.

gcc/rust/ChangeLog:

	* Make-lang.in: Compile the offset_of handler.
	* lang.opt: Add -frust-assume-builtin-offset-of option.
	* ast/rust-ast.h: Add has_str() for const_TokenPtr.
	* expand/rust-macro-builtins.cc: Map offset_of as builtin.
	* expand/rust-macro-builtins.h: Declare it.
	* expand/rust-macro-expand.cc (MacroExpander::expand_invoc): Add hack for calling builtin
	offset_of!().
	* resolve/rust-early-name-resolver-2.0.cc (Early::visit):  Likewise.
	* expand/rust-macro-builtins-offset-of.cc: New file.

gcc/testsuite/ChangeLog:

	* rust/compile/offset_of1.rs: New test.