Can't register mbed-client object successfully through WiFi

[cyliangtw]

Based on the up-to-date version, I can't register mbed-client object successfully through WiFi on K64F.
However, it could work fine based on the old version download on 07/18.

[ciarmcom]

ARM Internal Ref: IOTCLT-918

[yogpan01]

@geky Please check this.

[geky]

Looking into this more, but it looks like this issue only occurs when mbed-trace is enabled

[maclobdell]

also looks like compiling with gcc_arm works, but with armcc compiler it does not.

[geky]

This requires more testing, but I believe ARMmbed/mbed-os#2530 should solve this issue.