[vm/win] Release mappable snapshot once done loading.

Bug: flutter/flutter#146375
Bug: #55417
Change-Id: I7523579031091f9e75b2939c570d29338f33e176
TEST=ci, manually
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/361985
Reviewed-by: Ryan Macnak <[email protected]>
Commit-Queue: Alexander Aprelev <[email protected]>

Author: aam

runtime/bin/elf_loader.cc

@@ -289,6 +289,8 @@ bool LoadedElf::Load() {
   CHECK(ReadSectionStringTable());
   CHECK(ReadSections());
 
+  mappable_.reset();
+
   return true;
 }