RFR: 8286876: NMT.test_unaliged_block_address_vm_assert fails if using clang toolchain

[Johan Sjölen]

This fixes a bug by removing some UB which Clang used in order to make a test fail. Specifically, Clang optimized the `is_aligned` call in ` MallocHeader::assert_block_integrity` to `true`, because an unaligned `this` is  UB. This is fixed by making a static method, essentially. The new code liberally uses `reinterpret_cast<uintptr_t>()`, this is a specifically mentioned as legal use on https://en.cppreference.com/w/cpp/language/reinterpret_cast .

**The idea is essentially:** Make sure the pointer looks OK before casting it to a `MallocHeader*`.

I also changed the definition of `malloc_header`:

```c++
// old
    return (MallocHeader*)((char*)memblock - sizeof(MallocHeader));
// new
    return &(((MallocHeader*)memblock)[-1]);


The previous definition was UB, because you shouldn't cast from a pointer with smaller alignment to one with larger alignment requirements.

I also made `MallocHeader::print_block_on_error` static, as the `this` argument was always equal to the `bad_address`.

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Commit messages:
 - Fix the bug

Changes: https://git.openjdk.org/jdk/pull/11465/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11465&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8286876
  Stats: 62 lines in 6 files changed: 30 ins; 18 del; 14 mod
  Patch: https://git.openjdk.org/jdk/pull/11465.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11465/head:pull/11465

PR: https://git.openjdk.org/jdk/pull/11465