syscalls: use ARRAY_SIZE(x) instead of hard code define

Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Add support for cross building using CROSS_COMPILE

Similarly to the kernel.  CROSS_COMPILE is optional, when specified it's
a prefix, eg. myarch-linux-gnu-

We still allow overriding CC, separately, or in addition to setting
CROSS_COMPILE.

There is also support for cross configuring, because at the moment all
the configure tests only need to build, they're not run.
Ignore vim swap files
Fix compile error in net/bpf.c for non x86

In bpf.c there is an architecture check with cases for i386, x86-64 and
"other". But the other case typos TRUE_ARCH, leading to a build error.
Sanity check the syscall number in print_syscall_name()

In case it's out of bounds, causing us to segfault.