enable use of <ctime> with micro_time with -DTF_LITE_USE_CTIME

[advaitjain]

This change:

• uses a based micro_time implementation by default for ohost-builds (previously needed TAGS=posix)
• sets -DTF_LITE_USE_CTIME in the target makefiles for cross-compilation (instead of having to specialize micro_time.cc in the target folder).

Tested that we see the benchmark timing with the following commands:

Linux:

make -f tensorflow/lite/micro/tools/make/Makefile -j8 test_keyword_benchmark

Output:

InitializeKeywordRunner() took 35 ticks (0 ms)
KeywordRunNIerations(1) took 22 ticks (0 ms)
KeywordRunNIerations(10) took 136 ticks (0 ms)

Linux + xtensa_hifimini

make -f tensorflow/lite/micro/tools/make/Makefile -j8 TARGET=xtensa_hifimini OPTIMIZED_KERNEL_DIR=xtensa_hifimini XTENSA_CORE=mini1m1m_RG test_keyword_benchmark

Output:

InitializeKeywordRunner() took 1389000 ticks (1389 ms)
KeywordRunNIerations(1) took 89318 ticks (89 ms)
KeywordRunNIerations(10) took 892739 ticks (892 ms)

Linux + hexagon: untested

Fixes: http://b/172372039

[google-ml-butler]

Thanks for contributing to TensorFlow Lite Micro.

To keep this process moving along, we'd like to make sure that you have completed the items on this list:

• Read the contributing guidelines for TensorFlow Lite Micro
• Created a TF Lite Micro Github issue
• Linked to the issue from the PR description

We would like to have a discussion on the Github issue first to determine the best path forward, and then proceed to the PR review.