Fix initial SP for new Silicon Labs targets on ARMCC

[stevew817]

Description

EFR32MG1, EFR32MG12 and EFM32PG12 didn't have a fixed ARMCC linker script yet.

Pull request type

[ X ] Fix
[ ] Refactor
[ ] New target
[ ] Feature
[ ] Breaking change

[stevew817]

CC @screamerbg @ashok-rao

[cmonr]

👍

[0xc0170]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 1608
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/6488/

Triggering tests

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

[ashok-rao]

LGTM @stevew817 . Thanks!

[screamerbg]

LGTM!

[mbed-ci]

Exporter Build : SUCCESS

Build number : 1240
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/6488/

[mbed-ci]

Test : SUCCESS

Build number : 1393
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/6488/1393

[adbridge]

@stevew817 For future reference please do not remove the PR proforma . Just add your description in the section provided and make sure you select the PR type in the list (by just adding a X to the relevant type). Also the description should better explain what the PR is doing. Thanks.
I have added back the proforma to show you how it should look.

[mbed-ci]

Exporter Build : FAILURE

Build number : 1353
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/6488/