Fix UB when long type is narrower than mantissa_storage_type
Fix UB when long type is narrower than mantissa_storage_type

Use mantissa_storage_type instead of simple literal for value of
one to avoid undefined behaviour when long type (unsigned long,
which 0x1ul is being resoluted to) is narrower than bit size of
mantissa_storage_type (typically 64).

Fixes #1

aclex/floaxie,aclex/floaxie