IAR: Fix for #7662, only massage the error decode URL for the IAR .xcl

[TTornblom]

Description

tools/toolchains/iar.py:
Replace "//" sequence in get_compile_options() for_asm when generating the .xcl file.
This eliminates the need to escape this sequence in the .json files, which upsets other toolchains.

Tested with IAR and GCC_ARM toolchains with the mbed-os-example-blinky example.

platform/mbed_lib.json:
Removed the "\" escape sequence

Pull request type

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

[cmonr]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 3087
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/8017/

Triggering tests

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

[cmonr]

/morph mbed2-build

[cmonr]

/morph mbed2-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 2697
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/8017/

[cmonr]

Will rerun mbed2-build test shortly. Meant to run a different PR.

[mbed-ci]

Test : SUCCESS

Build number : 2875
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/8017/2875

[cmonr]

/morph mbed2-build

[cmonr]

👍