core: riscv: apply "-march" and "-mabi" options to assembler

Update platform-aflags-generic to include the -march option. Without
specifying -march, the assembler will enable the C extension by default
and generate compressed instructions, even if CFG_RISCV_ISA_C=n.

Signed-off-by: Yu Chien Peter Lin <peterlin@andestech.com>
Reviewed-by: Alvin Chang <alvinga@andestech.com>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>

core/arch/riscv/riscv.mk

@@ -122,7 +122,7 @@ rv64-platform-cppflags += -DRV64=1 -D__LP64__=1
 rv32-platform-cppflags += -DRV32=1 -D__ILP32__=1
 
 platform-cflags-generic ?= -ffunction-sections -fdata-sections -pipe
-platform-aflags-generic ?= -pipe
+platform-aflags-generic ?= -pipe -march=$(riscv-isa) -mabi=$(riscv-abi)
 
 rv64-platform-cflags-generic := -mstrict-align $(call cc-option,)