Fix compile error with MBED_MEM_TRACING_ENABLED and ARMC6

[ccli8]

Description

This PR tries to fix the compile error. Take mbed-os-example-blinky for example. If I compile with:

mbed compile -m NUMAKER_PFM_NUC472 -t ARMC6 -DMBED_MEM_TRACING_ENABLED=1

I meet error with:

Compile [ 77.1%]: mbed_interface.c
Compile [ 77.3%]: mbed_mem_trace.cpp
Compile [ 77.6%]: mbed_poll.cpp
Compile [ 77.8%]: mbed_sdk_boot.c
Compile [ 78.1%]: mbed_retarget.cpp
[ERROR] .\mbed-os\platform\mbed_retarget.cpp:1390:10: fatal error: 'reent.h' file not found
#include <reent.h>
         ^~~~~~~~~
1 error generated.

Pull request type

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

[0xc0170]

/morph build

[0xc0170]

Our CI runs still with ARMC5 (I recall if you do a PR for ARMC6 feature branch then it would run for that one). Is that good to land this one on master (how to test?) @theotherjimmy please review

[ccli8]

@0xc0170 I didn't send a PR for ARMC6 feature branch before. This error was found on my port for NUMAKER_PFM_M2351 target.

[theotherjimmy]

@0xc0170 I ran it locally. works. No warnings 😮

[theotherjimmy]

It's best to go to master with this.

[theotherjimmy]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 1874
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/6762/

Triggering tests

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

[mbed-ci]

Exporter Build : FAILURE

Build number : 1522
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/6762/

[mbed-ci]

Test : FAILURE

Build number : 1688
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/6762/1688

[adbridge]

Both the test failures appear to be due to infrastructure issues. Re-running:
/morph test
/morph export-build

[mbed-ci]

Test : SUCCESS

Build number : 1693
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/6762/1693

[mbed-ci]

Exporter Build : SUCCESS

Build number : 1528
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/6762/