bpf: CO-RE builtins support tests.

bpf: CO-RE builtins support tests.

This patch adds tests for the following builtins:
  __builtin_preserve_enum_value
  __builtin_btf_type_id
  __builtin_preserve_type_info

gcc/testsuite/ChangeLog:

	* gcc.target/bpf/core-builtin-enumvalue.c: New test.
	* gcc.target/bpf/core-builtin-enumvalue-errors.c: New test.
	* gcc.target/bpf/core-builtin-enumvalue-opt.c: New test.
	* gcc.target/bpf/core-builtin-fieldinfo-const-elimination.c: New test.
	* gcc.target/bpf/core-builtin-fieldinfo-errors-1.c: Changed.
	* gcc.target/bpf/core-builtin-fieldinfo-errors-2.c: Changed.
	* gcc.target/bpf/core-builtin-type-based.c: New test.
	* gcc.target/bpf/core-builtin-type-id.c: New test.
	* gcc.target/bpf/core-support.h: New test.