runtime: rename _mkmalloc's module to avoid GOROOT conflict

Often we want to run _mkmalloc using a different toolchain. The
previous module path runtime/_mkmalloc would produce an ambiguous import
path error because the directory runtime/_mkmalloc exists in a Go 1.26+
toolchain. Rename the module to avoid the conflict.

Change-Id: Id73a3ca102081d8b51da58879aee851b6a6a6964
Reviewed-on: https://go-review.googlesource.com/c/go/+/761880
Reviewed-by: Michael Matloob <matloob@google.com>
Auto-Submit: Michael Matloob <matloob@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: matloob

src/runtime/_mkmalloc/go.mod

@@ -1,4 +1,4 @@
-module runtime/_mkmalloc
+module _mkmalloc
 
 go 1.24