toolchainfile.cmake.in: do not force the CMAKE_{C, CXX}_FLAGS to the cache

Fix #7280 [1]

When the FORCE option is passed to the set command, the variable is
added/updated in the CMake cache every single time CMake processes this
command.

Because the toolchainfile.cmake prepends architecture/toolchain flags
to the CMAKE_{C,CXX}_FLAGS, this makes the CFLAGS being updated in the
generated Makefiles each time one reconfigures its project. So it
forces the compilation of everything, even when nothing has changed.

[1] https://bugs.busybox.net/show_bug.cgi?id=7280



Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Tested-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
(tested the SimpleApp reproduction scenario described in the bug report)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>