Use TARGET_ARCH=riscv64 when TARGET=riscv

The supported targets are riscv64 and riscv64sf. Use the former when
building with a bare TARGET=riscv and it is the more common one.

Sponsored by: Netflix

Makefile

@@ -270,7 +270,7 @@ MK_META_MODE= no
 # exceptions.
 .if !defined(TARGET_ARCH) && defined(TARGET)
 # T->TA mapping is usually TARGET with arm64 the odd man out
-_TARGET_ARCH=	${TARGET:S/arm64/aarch64/}
+_TARGET_ARCH=	${TARGET:S/arm64/aarch64/:S/riscv/riscv64/}
 .elif !defined(TARGET) && defined(TARGET_ARCH) && \
     ${TARGET_ARCH} != ${MACHINE_ARCH}
 # TA->T mapping is accidentally CPUARCH with aarch64 the odd man out