builder, cgo, syscall: replace fixed-size array casts with unsafe.Slice

Several places used the (*[1 << N]T)(ptr)[:len:len] pattern to convert
C pointers to Go slices. This has a hardcoded size limit that can panic
if exceeded; RunTool hit this with >1024 linker arguments when many
files are embedded.

Replace all instances with unsafe.Slice, which handles any size.

src/syscall/syscall_libc.go

@@ -256,7 +256,7 @@ func Mmap(fd int, offset int64, length int, prot int, flags int) (data []byte, e
 	if addr == unsafe.Pointer(^uintptr(0)) {
 		return nil, getErrno()
 	}
-	return (*[1 << 30]byte)(addr)[:length:length], nil
+	return unsafe.Slice((*byte)(addr), length), nil
 }
 
 func Munmap(b []byte) (err error) {