MFC r321483 (by ngie):

Mask issues with duplicate definitions for __fnstcw, __fldenv, and
__fldcw on i386 by ignoring -Wmacro-redefined.

This is a bandaid until the code is fixed and will be reverted before
MFC.

This unbreaks the tinderbox build for i386. Although the head commit
log suggests this would not be MFC'd, it nevertheless is being MFC'd as
the commit that enabled the warnings that this silences was MFC'd
without any proper fix for the warnings (head r321455, stable/11
r323299).