softfloat: fix {min, max}nummag for same-abs-value inputs

softfloat: fix {min, max}nummag for same-abs-value inputs

Before 8936006 ("fpu/softfloat: re-factor minmax", 2018-02-21),
we used to return +Zero for maxnummag(-Zero,+Zero); after that
commit, we return -Zero.

Fix it by making {min,max}nummag consistent with {min,max}num,
deferring to the latter when the absolute value of the operands
is the same.

With this fix we now pass fp-test.

Reviewed-by: Alex Bennée <[email protected]>
Signed-off-by: Emilio G. Cota <[email protected]>
Signed-off-by: Alex Bennée <[email protected]>
Message-id: 20180413140334[email protected]
Signed-off-by: Peter Maydell <[email protected]>