Fix single-threaded apps for IAR v8 builds

[TacoGrandeTX]

For single-threaded apps __iar_Initlocks() is not required and must not be called because the proper IAR library is brought in only if --threaded_lib is defined in the linker command.

Description

If one doesn't link with --threaded_lib then an IAR run-time library is brought in which doesn't include __iar_Initlocks() and a linker error is generated when building:

Error[Li005]: no definition for "__iar_Initlocks" [referenced from C:\_nrf\ble_
          project\BUILD\NRF52840_DK\IAR-TOOLCHAINS_PROFILE\mbed-os\r
          tos\TARGET_CORTEX\TOOLCHAIN_IAR\mbed_boot_iar.o]

So calls to __iar_Initlocks() now need to be protected by "MBED_RTOS_SINGLE_THREAD".

Pull request type

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

Reviewers

Release Notes

This was discovered on an internal BLE project which used the following toolchains_profile.json:

"IAR": {
    "common": [
        "--no_wrap_diagnostics", "-e",
        "--diag_suppress=Pa050,Pa084,Pa093,Pa082", "-Ohz", "-DMBED_RTOS_SINGLE_THREAD", "--enable_restrict"],
    "asm": [],
    "c": ["--vla"],
    "cxx": ["--guard_calls", "--no_static_destruction", 
            "--no_exceptions", "--no_rtti", "--no_static_destruction"],
    "ld": ["--skip_dynamic_initialization", "--inline", "--merge_duplicate_sections", "--no_exceptions", "--basic_heap"]
}

[TacoGrandeTX]

@paul-szczepanek-arm This will fix the build issue you experienced.

[ciarmcom]

@TacoGrandeTX, thank you for your changes.
@ARMmbed/mbed-os-core @ARMmbed/mbed-os-maintainers please review.

[0xc0170]

Good catch !

[cmonr]

@SenRamakri Mind taking a quick look?

[0xc0170]

CI started

[mbed-ci]

Test run: SUCCESS

Summary: 13 of 13 test jobs passed
Build number : 1
Build artifacts