[PATCH, libgfortran] Fix PR21875 (FM111.f) BN and BZ format specifiers

[Jerry DeLisle]

Jerry DeLisle wrote:
> This patch implements the BZ and BN format specifiers correctly.
> 
> Bubblestrapped and regression tested.  Tested with original NIST FM111.f 
> and attached test case.
> 
> After this there are three remaining failures in NIST being tracked in 
> PR21875.
> 
> OK to commit to mainline?  4.0?
> 
Hold up, found a tweak in the exponent needed. (and test case)

Will resubmit after some sleep.

Jerry