tests: IRIX 6.2 cc can't compile -0.0 into .data

tests: IRIX 6.2 cc can't compile -0.0 into .data

* tests/test-ceill.c (minus_zero): Compute -0.0L at runtime,
rather than at compile-time.
* tests/test-floorl.c (minus_zero): Likewise.
* tests/test-frexpl.c (minus_zero): Likewise.
* tests/test-isnan.c (minus_zerol): Likewise.
* tests/test-isnanl.h (minus_zero): Likewise.
* tests/test-ldexpl.c (minus_zero): Likewise.
* tests/test-roundl.c (minus_zero): Likewise.
* tests/test-signbit.c (minus_zerol): Likewise.
* tests/test-snprintf-posix.h (minus_zerol): Likewise.
* tests/test-sprintf-posix.h (minus_zerol): Likewise.
* tests/test-truncl.c (minus_zero): Likewise.
* tests/test-vasnprintf-posix.c (minus_zerol): Likewise.
* tests/test-vasprintf-posix.c (minus_zerol): Likewise.
Reported by Tom G. Christensen and Nelson H. F. Beebe.

Signed-off-by: Eric Blake <ebb9@byu.net>