Cleanup ISA string setting

Cleanup ISA string setting

This patch cleanup the MARCH string passing to both compiler and
assembler.  Note that the CFLAGS should not contain "fd" before we
have mechnisms like kernel_fpu_begin/end in other architectures.

Signed-off-by: Alan Kao <alankao@andestech.com>
Cc: Greentime Hu <greentime@andestech.com>
Cc: Vincent Chen <vincentc@andestech.com>
Cc: Zong Li <zong@andestech.com>
Cc: Nick Hu <nickhu@andestech.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>