[compiler-rt][builtins] Skip building (b)float16 support on i386-freebsd

Since bfloat16 and float16 support is not available for i386-freebsd,
the `truncdfbf2.c` and `truncsfbf2.c` builtin sources should be skipped
when targeting that platform, and `COMPILER_RT_HAS_FLOAT16` should not
be defined.

However, the CMake configuration stage runs its tests with the default
target, which normally is amd64-freebsd, so it will detect both bfloat16
and float16 support.

Move adding of the `COMPILER_RT_HAS_FLOAT16` define to the `foreach()`
loop where all the supported architectures are handled, and do not
enable it when targeting i386-freebsd.

Also remove the bfloat16 sources from the `i386_SOURCES` list, when
targeting i386-freebsd.

Differential Revision: https://reviews.llvm.org/D136044