Align Mbed CLI 2 to Mbed CLI 1 on C/C++ language standard

[ccli8]

Summary of changes

Mbed CLI 2 doesn't specify C/C++ language standard -std=gnu11 and -std=gnu++14. This PR tries to align Mbed CLI 2 to Mbed CLI 1 on C/C++ language standard, so that it can fix the error on Arm Compiler 6.21:

Error: ISO C++17 does not allow 'register' storage class specifier

---

Pull request type

[x] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

---

[0xc0170]

I need to check, dont recall why we did not add gnu, if it was overlooked.

[0xc0170]

Looking at the issue , it has been discussed #12232 and there are lot of other referenced issues.

I dont feel confident adding std standard as it is, I would rather focus on where is this coming from:
Error: ISO C++17 does not allow 'register' storage class specifier - that should be fixed in the codebase.

[ccli8]

The issue Error: ISO C++17 does not allow 'register' storage class specifier can reproduce following:

1. Change armclang to newer version e.g. 6.21
2. Change mbed-os to 6.17.0
3. In mbed-os-example-blinky, compile with Mbed CLI 2:

mbed-tools compile -m K64F -t ARM

[0xc0170]

The issue Error: ISO C++17 does not allow 'register' storage class specifier can reproduce following:

1. Change armclang to newer version e.g. 6.21
2. Change mbed-os to 6.17.0
3. In mbed-os-example-blinky, compile with Mbed CLI 2:

mbed-tools compile -m K64F -t ARM

I wanted to see what component is having an issue. I would guess it's cmsis as I saw a report some time ago related to this register.

I would rather fix the component.

[0xc0170]

Let's leave this in mbed tools as it is (application can choose what it requires rather than enforcing this in the tooling). I'll close this.

If there is an issue with register and std, we should fix but differently.