Most arguments were quoted, but some weren't, causing problems if the
arguments contained whitespace.

In particular, the _STL_EXTRA_DISABLED_WARNINGS value takes spaces and
CMake's NASM support applies all add_definitions lines to NASM. The -D
flag is missing quotes, so projects using NASM and setting
_STL_EXTRA_DISABLED_WARNINGS break in the Visual Studio generator.

Likewise, the -o flag is missing quotes, which means filenames with
spaces do not work.

(The -U flag is unlikely to need quotes, but include them for
consistency.)

Extend the existing VSNASM test to cover these cases.