libgcc: enable DFP for AArch64

DFP support on AArch64 relies on libgcc, so enable its DFP routines
for all AArch64 targets.

2022-03-31  Christophe Lyon  <christophe.lyon@arm.com>

	libgcc/
	* config.host: Add t-dfprules to AArch64 targets.