xtensa: generate uapi header and syscall table header files

xtensa: generate uapi header and syscall table header files

System call table generation script must be run to gener-
ate unistd_32.h and syscall_table.h files. This patch will
have changes which will invokes the script.

This patch will generate unistd_32.h and syscall_table.h
files by the syscall table generation script invoked by
xtensa/Makefile and the generated files against the removed
files must be identical.

The generated uapi header file will be included in uapi/-
asm/unistd.h and generated system call table header file
will be included by kernel/syscall.c file.

Signed-off-by: Firoz Khan <firoz.khan@linaro.org>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>