port-mips/50723: FPU handling in MIPS is broken

[macallan%netbsd.org@localhost]

>Number:         50723
>Category:       port-mips
>Synopsis:       FPU handling in MIPS is broken
>Confidential:   no
>Severity:       critical
>Priority:       high
>Responsible:    port-mips-maintainer
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Fri Jan 29 01:50:00 +0000 2016
>Originator:     Michael
>Release:        7.99.26
>Organization:
TNF
>Environment:
NetBSD ci20 7.99.26 NetBSD 7.99.26 (CI20) #125: Thu Jan 28 20:31:33 EST 2016  ml@blackbush:/home/build/obj_evbmips32/sys/arch/evbmips/compile/CI20 evbmips

>Description:
FPU registers get corrupted, looks exactly like powerpc/50037, can be triggered with the same test program.

I saw this only on CI20 ( MIPS32R2 ) so far, will check sgimips later.
>How-To-Repeat:
run the test program from powerpc/50037, get this:
~/fpu# cc -pipe -march=mips32r2   -o main main.c
~/fpu# ./main 
diff: 2879.229316 - 2.000000 3.000000 6.000000 6.000000 2879.229316
 00 f8 ec 68 75 7e a6 40
diff: 2155.314669 - 2.000000 3.000000 6.000000 6.000000 2155.314669
 00 3c 4a 1c a1 d6 a0 40
diff: 2886.203915 - 2.000000 3.000000 6.000000 6.000000 0.000000
 00 80 7b 67 68 8c a6 40
diff: 0.001020 - 2.000000 3.000000 6.000000 6.000000 0.001020
 28 67 8e 04 f3 b5 50 3f
diff: 4717.457986 - 2.000000 3.000000 6.000000 6.000000 4717.457986
 00 02 98 3e 75 6d b2 40
diff: 5544.622506 - 2.000000 3.000000 6.000000 6.000000 5544.622506
 00 c6 89 5c 9f a8 b5 40
diff: 5965532835022.678711 - 2.000000 3.000000 5965532835028.678711 6.000000 -5965532835022.678711
 b7 3a b3 dc d5 b3 95 42

>Fix: