linux-user: Add support for syncfs() syscall

This patch implements Qemu user mode syncfs() syscall support. Syscall
syncfs() syncs the filesystem containing file determined by the open
file descriptor passed as the argument to syncfs().

The implementation consists of a straightforward invocation of host's
syncfs(). Configure and strace support is included as well.

Signed-off-by: Aleksandar Markovic <aleksandar.markovic@imgtec.com>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>