x86/entry: Move max syscall number calculation to syscallhdr.sh

Instead of using an array in asm-offsets to calculate the max syscall
number, calculate it when writing out the syscall headers.

Signed-off-by: Brian Gerst <brgerst@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20200313195144.164260-9-brgerst@gmail.com