[vm] Increase alignment in VM-generated ELF to 16k.

ARM64 Mac has 16k pages, so we need snapshot parts to be 16k aligned for our ELF loader to load them via mmap.

TEST=ci
Bug: #42773
Change-Id: Ifa3d70e453ea72883d821e48cfc76b0d983391f2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/193821
Commit-Queue: Ryan Macnak <[email protected]>
Reviewed-by: Siva Annamalai <[email protected]>

Author: rmacnak-google

runtime/vm/elf.h

@@ -32,7 +32,7 @@ class Elf : public ZoneAllocated {
 
   Elf(Zone* zone, BaseWriteStream* stream, Type type, Dwarf* dwarf = nullptr);
 
-  static constexpr intptr_t kPageSize = 4096;
+  static constexpr intptr_t kPageSize = 16 * KB;
   static constexpr uword kNoSectionStart = 0;
 
   bool IsStripped() const { return dwarf_ == nullptr; }