kernel: don't wildcard *.sh for linux_syscall_numbers.h

During a parallel build it's possible that $(wildcard *.sh) evaluates to
"regen.sh linux_syscall_numbers.h.1234.sh", where 1234 is the PID from a
currently executing regen.sh and .sh is the architecture name.  This
temporary file then disappears resulting in "file not found" errors when
the dependencies are evaluated.

The wildcard isn't actually needed as the only script is regen.sh, so
hard-code that into the dependencies (and use $< to reduce duplication).

Signed-off-by: Ross Burton <ross.burton@intel.com>