make gcc-specific headers (intrinsics, etc.) available with wrapper

they are intentionally listed after the libc include directory so that
the gcc float.h, etc. don't get used in place of the libc ones.

tools/musl-gcc.specs.sh

@@ -5,10 +5,10 @@ cat <<EOF
 %rename cpp_options old_cpp_options
 
 *cpp_options:
--nostdinc -isystem $incdir %(old_cpp_options)
+-nostdinc -isystem $incdir -isystem include%s %(old_cpp_options)
 
 *cc1:
-%(cc1_cpu) -nostdinc -isystem $incdir 
+%(cc1_cpu) -nostdinc -isystem -isystem include%s $incdir 
 
 *link_libgcc:
 -L$libdir -L .%s