Support Graviton c7g and c6gn instance types on AWS 7.0.8

This enhancement allows FGT-ARM64-AWS to work in Graviton3 c7g and Graviton 2 c6gn instance types.

The following shows the described deployment on the AWS EC2 console and FortiOS GUI:

To view the VM device information in the CLI:

# get system status
Version: FortiGate-ARM64-AWS v7.0.7,build0408,220914 (interim)
Virus-DB: 1.00000(2018-04-09 18:07)
Extended DB: 1.00000(2018-04-09 18:07)
Extreme DB: 1.00000(2018-04-09 18:07)
AV AI/ML Model: 0.00000(2001-01-01 00:00)
IPS-DB: 6.00741(2015-12-01 02:30)
IPS-ETDB: 6.00741(2015-12-01 02:30)
APP-DB: 6.00741(2015-12-01 02:30)
INDUSTRIAL-DB: 6.00741(2015-12-01 02:30)
IPS Malicious URL Database: 1.00001(2015-01-01 01:01)
Serial-Number: FGVM04TM21011085
License Status: Valid
License Expiration Date: 2022-10-22
VM Resources: 4 CPU/4 allowed, 15755 MB RAM
Log hard disk: Available

# diagnose hardware sysinfo cpu
processor       : 0
BogoMIPS        : 2100.00
Features        : fp asimd evtstrm aes pmull sha1 sha2 crc32 atomics fphp asimdhp cpuid asimdrdm jscvt fcma lrcpc dcpop sha3 sm3 sm4 asimddp sha512 sve asimdfhm dit uscat ilrcpc flagm
CPU implementer : 0x41
CPU architecture: 8
CPU variant     : 0x1
CPU part        : 0xd40
CPU revision    : 1