cmake: gcc.cmake: allow users to influence CMAKE_REQUIRED_FLAGS
cmake: gcc.cmake: allow users to influence CMAKE_REQUIRED_FLAGS

A GCC-based toolchain may require additional, toolchain-specific
values in CMAKE_REQUIRED_FLAGS to perform compiler checks properly,
but gcc.cmake clobbers any values the user provides.

Preserve them instead, allowing users to give their own compiler
checking flags at generation time.

The details for the particular issue that inspired this are described
in https://github.com/pulp-platform/pulpino/issues/240.

Signed-off-by: Marti Bolivar <fd5da480b758a78208111e6ae5c6806e1f887137@foundries.io>

finikorg/zephyr,nashif/zephyr,Vudentz/zephyr,galak/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,explora26/zephyr,Vudentz/zephyr,Vudentz/zephyr,nashif/zephyr,ldts/zephyr,finikorg/zephyr,nashif/zephyr,galak/zephyr,nashif/zephyr,ldts/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,explora26/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,ldts/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr