[vm/bytecode] Remove alignment of sections in bytecode

Previously, before switching to compact encoding of bytecode
instructions, we had to align bytecode instructions by 4.
Since that time sections of bytecode binary were aligned.

However, this alignment doesn't work well in AOT mode if dill file
contains both bytecode and AST, as other metadata (such as obfuscation
prohibitions) may precede bytecode metadata and may make the whole
bytecode binary misaligned.

This results in incorrect calculation of section offsets,
failed assertion

  assert(writer.offset - start == section.offset);

and VM may fail to load bytecode binary with error

  bytecode_reader.cc: 1368: error: unreachable code

In order to fix these problems, extra alignment of bytecode
sections is removed.

Change-Id: Iaa3b7c46999615aaa446781a7018db409261ead8
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/123686
Reviewed-by: Régis Crelier <regis@google.com>
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Commit-Queue: Alexander Markov <alexmarkov@google.com>