math/R: Fix dependency condition for devel/openmp

libomp is provided by

• LLVM, which gets pulled in with FLANG
• gcc

So, we *must* only depend on devel/openmp when

• the OPENMP knob is on
• FLANG is off (guaranteed to be off for all archs except amd64)
• clang is the C compiler.

But, we cannot modify LIB_DEPENDS after CHOSEN_COMPILER_TYPE is defined, so
for now, limit the OPENMP knob for i386 and amd64 where clang is the
default compiler.

This will be moot after 12.1 when openmp will be in base.

PR: 240279
Reported by: pkubaj
Reviewed by: pkubaj