system_reset is MBED_NORETURN

[kjbracey]

Description

Follow-on from #8328 - system_reset is another noreturn function.

Pull request type

[ ] Fix
[X] Refactor
[ ] Target update
[ ] Functionality change
[ ] Docs update
[ ] Test update
[ ] Breaking change

[cmonr]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 3508
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/8584/

Triggering tests

/morph test
/morph export-build
/morph mbed2-build

[mbed-ci]

Test : SUCCESS

Build number : 3293
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/8584/3293

[mbed-ci]

Exporter Build : SUCCESS

Build number : 3127
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/8584/

[0xc0170]

@ARMmbed/mbed-os-core final approval needed