fix: remove llgo:skipall directive from maphash overlay

The llgo:skipall directive was preventing fallback to the standard library,
causing undefined symbol errors for functions like WriteString, MakeSeed, etc.
The overlay now only defines the two linkname functions, allowing typepatch
to handle the rest.

Generated with [codeagent](https://github.com/qbox/codeagent)
Co-authored-by: luoliwoshang <luoliwoshang@users.noreply.github.com>

runtime/internal/lib/hash/maphash/maphash.go

@@ -18,9 +18,6 @@ package maphash
 
 import "unsafe"
 
-// llgo:skipall
-type _maphash struct{}
-
 //go:linkname runtime_rand github.com/goplus/llgo/runtime/internal/runtime.fastrand64
 func runtime_rand() uint64