don't set TARGET_FLAGS for generic RISC-V targets

Setting the TARGET_FLAGS causes fortran detection to fail on Linux
unless you have an f77 compiler on your path.

Fixes: #5424

Author: markdryan

Makefile.prebuild

@@ -63,18 +63,6 @@ ifeq ($(TARGET), x280)
 TARGET_FLAGS = -march=rv64imafdcv_zba_zbb_zfh -mabi=lp64d
 endif
 
-ifeq ($(TARGET), RISCV64_ZVL256B)
-TARGET_FLAGS = -march=rv64imafdcv -mabi=lp64d
-endif
-
-ifeq ($(TARGET), RISCV64_ZVL128B)
-TARGET_FLAGS = -march=rv64imafdcv -mabi=lp64d
-endif
-
-ifeq ($(TARGET), RISCV64_GENERIC)
-TARGET_FLAGS = -march=rv64imafdc -mabi=lp64d
-endif
-
 all: getarch_2nd
 	./getarch_2nd  0 >> $(TARGET_MAKE)
 	./getarch_2nd  1 >> $(TARGET_CONF)