gnu.cfg: Remove duplicate define (#5931)

Author: orbitcowboy

cfg/gnu.cfg

@@ -75,7 +75,6 @@
   <define name="__builtin_nan(str)" value="nan(str)"/>
   <define name="__builtin_nanf(str)" value="nanf(str)"/>
   <define name="__builtin_nanl(str)" value="nanl(str)"/>
-  <define name="__builtin_signbit(value)" value="signbit(value)"/>
   <define name="__extension__" value=""/>
   <define name="__typeof__(T)" value="typeof(T)"/>
   <define name="__glibc_unlikely(x)" value="(x)"/>