bsd-user: Don't truncate the return value from freebsd_syscall

bsd-user: Don't truncate the return value from freebsd_syscall

System call return values on FreeBSD are in a register (which is spelled
abi_long in qemu). This was being assigned into an int variable which
causes problems for 64bit targets.

Resolves: https://github.com/qemu-bsd-user/qemu-bsd-user/issues/40
Signed-off-by: Doug Rabson <dfr@rabson.org>
Reviewed-by: Warner Losh <imp@bsdimp.com>
[ Edited commit message for upstreaming into qemu-project ]
Signed-off-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>