[vm/msvc] Clean up use of #if/#endif in macro argument.

MSVC(unlike clang) seems to have a problem with such a use:

```
ninja -t msvc -e environment.x64 --
"C:\src\depot_tools\win_toolchain\vs_files\20d5f2553f\VC\Tools\MSVC\14.26.28801\bin\Hostx64\x64/cl.exe"
/nologo /showIncludes /FC
@obj/runtime/vm/libdart_vm_jit.runtime_entry.obj.rsp /c
../../runtime/vm/runtime_entry.cc
/Foobj/runtime/vm/libdart_vm_jit.runtime_entry.obj
/Fdobj/runtime/vm/libdart_vm_jit_cc.pdb

C:\src\dart-sdk\sdk\runtime\vm\runtime_entry.cc(3683): error C2121: '#':
invalid character: possibly the result of a macro expansion
C:\src\dart-sdk\sdk\runtime\vm\runtime_entry.cc(3683): error C2059:
syntax error: 'if'
C:\src\dart-sdk\sdk\runtime\vm\runtime_entry.cc(3693): error C2121: '#':
invalid character: possibly the result of a macro expansion
C:\src\dart-sdk\sdk\runtime\vm\runtime_entry.cc(3693): error C2059:
syntax error: 'if'
```

TEST=build sdk on Windows with msvc(rather than clang)

Change-Id: I520b32bf38bc7880f45d93223779f10cbde59af6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/207041
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Alexander Aprelev <aam@google.com>