[libc][z/OS] Remove ASCII trick to fix EBDIC std::from_char (#116078)

This PR will fix the following lit in all EBCDIC variations on z/OS:
`std/utilities/charconv/charconv.from.chars/floating_point.pass.cpp`

The trick to test for `e` and `E` is working only in ASCII.
The fix is to simply test for both lower and upper case exponent letter
`e` and `E` respectfully.

GitOrigin-RevId: e25e8867348953c17fa0d0b79f43bde758ad8b37
1 file changed