arch/riscv: set the default float ABI based on ISA extensions

arch/riscv: set the default float ABI based on ISA extensions

This patch sets the default floating point ABI based on the ISA
extensions that have been selected rather than defaulting to soft
float.

For 64-bit:

ISA 'D' selects  lp64d
ISA 'F' selects  lp64f
Otherwise select lp64

For 32-bit:

ISA 'D' selects  ilp32d
ISA 'F' selects  ilp32f
Otherwise select ilp32

This change was proposed by Palmer Dabbelt at SiFive.

Signed-off-by: Mark Corbin <mark.corbin@embecosm.com>
Reviewed-by: Palmer Dabbelt <palmer@sifive.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>