[runtime] Fix MONO_ARCH_ENABLE_MONO_LMF_VAR

[runtime] Fix MONO_ARCH_ENABLE_MONO_LMF_VAR

Using MONO_ARCH_ENABLE_MONO_LMF_VAR requires that we have means of getting the address of a tls variable (which is always the case when using __thread, but not necessarily when using pthread). Make sure that using it is properly guarded.

In the future we might want to remove MONO_ARCH_ENABLE_LMF_VAR and always try to use a tls variable for the lmf. Additionally we should benchmark what combinations work best and stick to them on all platforms. (ex when using fast tls we might consider using lmf_get + lmf_set if possible, when using fallbacks we might always want to use the lmf_addr to void multiple calls, always use lmf_ir etc).