gnu.cfg: Added support for __builtin_bit_cast (danmar#7163)

Reference: https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html

Author: orbitcowboy

cfg/gnu.cfg

@@ -64,6 +64,7 @@
       <not-bool/>
     </arg>
   </function>
+  <define name="__builtin_bit_cast(type,value)" value="std::bit_cast&lt;type&gt;(value)"/>
   <define name="__bswap_constant_64(x)" value="bswap_64(x)"/>
   <define name="__builtin_bswap64(x)" value="bswap_64(x)"/>
   <define name="__bswap_64(x)" value="bswap_64(x)"/>