Fix bsd-user build errors after 8642c1b8

  LINK  sparc-bsd-user/qemu-sparc
bsd-user/main.o: In function `cpu_loop':
/home/sbruno/bsd/qemu/bsd-user/main.c:515: undefined reference to `cpu_sparc_exec'
c++: error: linker command failed with exit code 1 (use -v to see invocation)
gmake[1]: *** [Makefile:197: qemu-sparc] Error 1
gmake: *** [Makefile:204: subdir-sparc-bsd-user] Error 2

  LINK  i386-bsd-user/qemu-i386
bsd-user/main.o: In function `cpu_loop':
/home/sbruno/bsd/qemu/bsd-user/main.c:174: undefined reference to `cpu_x86_exec'
c++: error: linker command failed with exit code 1 (use -v to see invocation)
gmake[1]: *** [Makefile:197: qemu-i386] Error 1
gmake: *** [Makefile:204: subdir-i386-bsd-user] Error 2

Signed-off-by: Sean Bruno <sbruno@freebsd.org>
Message-id: 20160729160235.64525-1-sbruno@freebsd.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>