linux: Add LTS version 4.14.19

Announcements and change-logs are available online [1][2].

This release is built with GCC 7.3.0 instead of GCC 5.5.0, and therefore
contains full mitigation against Spectre v2, cf. commit f4c8184 (gcc:
Update version from 5.5.0 to 7.3.0).

Before:

```
/sys/devices/system/cpu/vulnerabilities/meltdown:Mitigation: PTI
/sys/devices/system/cpu/vulnerabilities/spectre_v1:Mitigation: __user pointer sanitization
/sys/devices/system/cpu/vulnerabilities/spectre_v2:Vulnerable: Minimal generic ASM retpoline
```

After:

```
/sys/devices/system/cpu/vulnerabilities/meltdown:Mitigation: PTI
/sys/devices/system/cpu/vulnerabilities/spectre_v1:Mitigation: __user pointer sanitization
/sys/devices/system/cpu/vulnerabilities/spectre_v2:Mitigation: Full generic retpoline
```

[1] https://lkml.org/lkml/2018/2/13/154
[2] https://cdn.kernel.org/pub/linux/kernel/v4.x/ChangeLog-4.14.19