nasm: Fix -MD operating mode
Previously -MD option didn't call for depend mode solely but
produced deps during active compilation phase allowing symbol
refereces processing as well.
Occasionally in commit 599a98 I made -MD option to handle deps
in a separate internal phase which caused problems like
| error: symbol references not supported in preprocess-only mode
leading to problems in building applications with nasm. Fix it
moving old behaviour back.
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>