libc: minimal: Add "prohibit_lto" flag to avoid problems with LTO

The compiler requires that much of the C library be built without using LTO
so that various symbols are available for use by generated code, including
things like memset and memcpy.

Add the "prohibit_lto" CMake target compiler property to avoid LTO when
activated.

Signed-off-by: Matthias Plöger <matthias.ploeger@phoenixcontact.com>