Fixed some conversion issues with Clang due to problematic undefined
behavior when casting a negative floating-point value to an integer
Fixed a bug in the library's software integer to floating-point
conversion function where a user's conversion exception function
returning H5T_CONV_UNHANDLED in the case of overflows would result in
incorrect data after conversion
Added configure checks for functions and macros related to _Float16
usage since some compilers expose the datatype but not the functions or
macros
Fixed a dt_arith test failure when H5_WANT_DCONV_EXCEPTION isn't defined
Fixed a few warnings from not explicitly casting some _Float16 variables
upwards
