build: simplify Makefile.target a bit, use just one rule for softmmu
On win*, we build QEMU_PROGW (GUI) and create a console app QEMU_PROG
from it, while on non-win*, we make only QEMU_PROG using the same
rules as used for QEMU_PROGW on win*. Make just one rule for building
main executable, and an additional rule for win* to make console app
from it. Also consolidate tests for $(QEMU_PROGW).
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
[Fix user-mode compilation. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>