[build] Mark __intel_new_proc_init with __libgcc rather than cdecl

The function __intel_new_proc_init() (called implicitly when building
using icc) is marked with __attribute__((cdecl)).  This breaks
building on x86_64, where cdecl is meaningless.

Fix by replacing with the existing __libgcc macro, which is already
defined to be "__attribute__((cdecl))" for i386 builds and empty for
x86_64 builds.

src/libgcc/icc.c

  * start of main().
  *
  */
-void __attribute__ (( cdecl )) __intel_new_proc_init ( void ) {
+void __libgcc __intel_new_proc_init ( void ) {
 	/* Do nothing */
 }