pw_enum: Fix code generation on Windows

The pw_enum code generator assumed POSIX paths and the platform default
text encoding, which broke it on Windows:

- Ninja writes paths with backslash separators. shlex.split() applies
  POSIX rules, which consumed those separators as escape characters, and
  the paths did not compare equal to the generator's own paths.
- GCC escapes the backslashes in the # line markers it emits.
- The default encoding and newline translation made generated headers
  and flag files differ from those produced on other platforms.
- Windows appends the PATHEXT extensions only when searching for a bare
  executable name, so the extensionless compiler path from GN failed to
  launch.

Re-enables pw_enum:enum_test in GN on Windows.

Change-Id: Iaec17b41611e5e441d4f1371f3d305b57d7078df
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/483690