utf8.h: Simplify UTF8_EIGHT_BIT_foo on EBCDIC

These macros were previously defined in terms of UTF8_TWO_BYTE_HI and
UTF8_TWO_BYTE_LO.  But the EIGHT_BIT versions can use the less general
and simpler NATIVE_TO_LATN1 instead of NATIVE_TO_UNI because the input
domain is restricted in the EIGHT_BIT.  Note that on ASCII platforms,
these both expand to the same thing, so the difference matters only on
EBCDIC.