Refer autoconfigured endian macro (#10572)

	Remove the case `RB_IO_BUFFER_HOST_ENDIAN` is not defined.
	---
	 include/ruby/io/buffer.h | 8 ++------
	 1 file changed, 2 insertions(+), 6 deletions(-)

	Check if macros are defined before using

	Assume macros with the same prefix would be defined together.
	---
	 include/ruby/io/buffer.h | 8 ++++----
	 1 file changed, 4 insertions(+), 4 deletions(-)