[phobos] bigint unittest failure on OSX

[Walter Bright]

Works on Windows and Ubuntu.
--------------------------------------------

Testing generated/osx/debug/unittest/std/bigint
core.exception.AssertError at std.bigint(476): unittest failure
----------------
5   bigint                              0x0000a918 onAssertErrorMsg + 68
6   bigint                              0x0000a95a onUnittestErrorMsg + 26
7   bigint                              0x00011f9d _d_unittestm + 45
8   bigint                              0x00001f3f void 
std.bigint.__unittest_fa
il(int) + 35
9   bigint                              0x00002716 void 
std.bigint.__unittest1()
 + 686
10  bigint                              0x00001ec7 void 
std.bigint.__modtest() +
 11
11  bigint                              0x0000ad25 extern (C) bool 
core.runtime.
runModuleUnitTests() + 45
12  bigint                              0x00005b47 
D6object10ModuleInfo7opApplyF
MDFKPS6object10ModuleInfoZiZi + 71
13  bigint                              0x0000ac16 runModuleUnitTests + 134
14  bigint                              0x00012306 extern (C) int 
rt.dmain2.main
(int, char**) + 38
15  bigint                              0x0001220a extern (C) int 
rt.dmain2.main
(int, char**) + 42
16  bigint                              0x00012197 main + 179
17  bigint                              0x00001ea9 start + 53
make[2]: *** [generated/osx/debug/unittest/std/bigint] Error 1
make[1]: *** [unittest] Error 2
make: *** [unittest] Error 2