Add CMAKE_C_FLAGS for clang-3 #921

Job	Run time
Package builder app	7s
Unit Tests	3s
Clean up previous branch jobs	8s
release_notes (aws-crt-python)	36s
compilers (clang-9)	49s
release_notes (aws-iot-device-sdk-python-v2)	6s
compilers (clang-10)	28s
cross_compile (linux-armv6)	10m 39s
compilers (clang-11)	39s
downstream (aws-c-mqtt)	10m 53s
cross_compile (linux-armv7)	10m 9s
compilers (gcc-7)	31s
downstream (aws-c-http)	3m 53s
Sanity Test (ubuntu-22.04)	1m 55s
cross_compile (linux-armv8)	9m 35s
downstream (aws-crt-python)	4m 29s
compilers (gcc-8)	37s
Sanity Test (macos-11)	3m 0s
downstream (aws-crt-cpp)	5m 40s
compilers (gcc-9)	27s
Sanity Test (macos-12)	2m 59s
swift (swift-5-ubuntu-x64)	44s
downstream (aws-crt-java)	8m 45s
compilers (gcc-10)	38s
Sanity Test (windows-2022)	2m 22s
swift (swift-5-centos-x64)	40s
compilers (gcc-11)	46s
downstream (aws-crt-nodejs)	4m 10s
swift (swift-5-al2-x64)	37s
aws-c-cal (manylinux1-x86)	1m 26s
aws-c-cal (manylinux1-x64)	1m 24s
aws-c-cal (manylinux2014-x86)	1m 31s
aws-c-cal (manylinux2014-x64)	2m 32s
aws-c-cal (manylinux2014-aarch64)	23m 0s
aws-c-cal (musllinux-1-1-aarch64)	24m 46s
aws-c-cal (musllinux-1-1-x64)	2m 37s
std-compat (gcc-8, c++11)	7m 7s
std-compat (gcc-8, c++14)	5m 20s
std-compat (gcc-8, c++17)	5m 33s
std-compat (gcc-8, c++2a)	7m 9s
std-compat (clang-9, c++11)	5m 17s
std-compat (clang-9, c++14)	6m 56s
std-compat (clang-9, c++17)	5m 6s
std-compat (clang-9, c++2a)	5m 58s
All Sanity Tests passed	1s
	3h 12m 8s

Provide feedback