Use coprocessor mnemonic to generate VFP instructions

In some configurations we don't compile the library with -mfpu=vfp* but we still
want to provide on-demand unwinding support for VFP, so also use the stc/ldc
coprocessor mnemonics to generate the f{st,ld}mia{d,x} instructions.

Change-Id: I0fd7401cbeb1979d0903efa5547d112a1cf24e4b