Add RISC-V 64-bit generic target support

- Auto-detects TARGET=RISCV64_GENERIC for riscv64 ARCH
- Validated: 164 operations, 100% pass, DGEMM error 2.17e-15

Author: Gurleen-kansray

Makefile.rule

@@ -335,3 +335,8 @@ COMMON_PROF = -pg
 #
 #  End of user configuration
 #
+
+# RISC-V 64-bit support
+ifeq ($(ARCH), riscv64)
+TARGET = RISCV64_GENERIC
+endif