CMakeLists.txt: parse zephyr_stdint.h after compiler specific flags

The zephyr_stdint.h file enforces Zephyr specific policies on the
compilation environment. Let's give compilers a chance to provide
definitions of their own via TOOLCHAIN_C_FLAGS prior the inclusion
of zephyr_stdint.h.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>