Build error on RISC-V architecture.

Does blastem has not supported RISC-V?

Logs:

make: Entering directory '/build/libretro-blastem/src/libretro-blastem'
make libblastem.so OS=Linux CC=cc CPU=x86_64 LIBRETRO=1
make[1]: Entering directory '/build/libretro-blastem/src/libretro-blastem'
cc -O2 -flto -march=rv64gc -mabi=lp64d -O2 -pipe -fno-plt -fexceptions         -Wp,-D_FORTIFY_SOURCE=3 -Wformat -Werror=format-security         -fstack-clash-protection         -fno-omit-frame-pointer -g -ffile-prefix-map=/build/libretro-blastem/src=/usr/src/debug/libretro-blastem -flto=auto -std=gnu99 -Wreturn-type -Werror=return-type -Werror=implicit-function-declaration -Wno-unused-value  -Wpointer-arith -Werror=pointer-arith -DHAS_PROC -DHAVE_UNISTD_H -DX86_64 -m64 -fpic -DIS_LIB -c -o libblastem.o libblastem.c
cc: error: unrecognized command-line option ‘-m64’
make[1]: *** [Makefile:366: libblastem.o] Error 1
make[1]: Leaving directory '/build/libretro-blastem/src/libretro-blastem'
make: *** [Makefile.libretro:61: core] Error 2
make: Leaving directory '/build/libretro-blastem/src/libretro-blastem'

Start with the command in the build log and inspect Makefile:366 and Makefile.libretro:61 to trace where the x86_64-specific flags are added. Compare that configuration with the RISC-V compiler invocation and verify the libretro target builds successfully for RISC-V without the reported unsupported option.