ARM: correct assertion for long-calls on WoA

COFF/PE, so the relocation model is never static.  Loosen the assertion
accordingly.  The relocation can still be emitted properly, as it will be
converted to an IMAGE_REL_ARM_ADDR32 which will be resolved by the loader
taking the base relocation into account.  This is necessary to permit the
emission of long calls which can be controlled via the -mlong-calls option in
the driver.