[arch][riscv64] fix build with older toolchains (gcc 7.5.0)

When building fpu variant, use -march=rv64imafdc instead of rc64gc
since some older compilers and/or mainline do not understand the
equivalence when selecting libgcc.

This is only an issue on rv64 due to the need to pick out the medany
variant of libgcc.

arch/riscv/rules.mk

@@ -152,7 +152,10 @@ else ifeq ($(SUBARCH),64)
     GLOBAL_DEFINES += IS_64BIT=1
 
     ifeq (true,$(call TOBOOL,$(RISCV_FPU)))
-        ARCH_COMPILEFLAGS := -march=rv64gc -mabi=lp64d -mcmodel=medany
+        # HACK: use rv64imafdc instead of the equivalent rv64gc due to
+        # older toolchains not supporting the mapping of one to the other
+        # when selecting libgcc.
+        ARCH_COMPILEFLAGS := -march=rv64imafdc -mabi=lp64d -mcmodel=medany
     else
         ARCH_COMPILEFLAGS := -march=rv64imac -mabi=lp64 -mcmodel=medany
     endif