Makefile: Add option to use toolchain default ABI and ISA string

When PLATFORM_RISCV_ABI and PLATFORM_RISCV_ISA are not specified,
we force "-mabi=lp64 -march=rv64gc" for RV64 and force "-mabi=ilp32
-march=rv32gc" for RV32. This can prevent users from using the
toolchain default "-mabi" and "-march" options.

To allow using toolchain defaults, we add compile-time option
PLATFORM_RISCV_TOOLCHAIN_DEFAULT which when enabled forces the
top-level makefile to use toolchain default ABI and ISA string.

To enable the option, pass "PLATFORM_RISCV_TOOLCHAIN_DEFAULT=1"
to top-level make.

Reported-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>