BaseTools/tools_def.template: Update -march parameter for RISC-V

Replace rv64imafdc with rv64gc so that it works for gcc 12.

"g" means imafd in gcc < 12 and imafd_zifencei_zicsr in gcc >= 12.
So, replacing rv64imafdc with rv64gc will work for both gcc <12
and gcc >=12.

Cc: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Yuwei Chen <yuwei.chen@intel.com>
Cc: Andrei Warkentin <andrei.warkentin@intel.com>
Cc: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Signed-off-by: Sunil V L <sunilvl@ventanamicro.com>
Tested-by: Gerd Hoffmann <kraxel@redhat.com>
Tested-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>