Fall back to RV64 compiler if no RV32 compiler and vice versa

Author: aswaterman

isa/Makefile

@@ -46,6 +46,15 @@ default: all
 # Build rules
 #--------------------------------------------------------------------
 
+RISCV_PREFIX ?= riscv$(XLEN)-unknown-elf-
+ifeq ($(shell which $(RISCV_PREFIX)gcc 2> /dev/null),)
+	ALT_XLEN := $(if $(call eq,$(XLEN),64),32,64)
+  RISCV_PREFIX := riscv$(ALT_XLEN)-unknown-elf-
+  ifeq ($(shell which $(RISCV_PREFIX)gcc 2> /dev/null),)
+    $(error couldn't find gcc)
+  endif
+endif
+
 RISCV_PREFIX ?= riscv$(XLEN)-unknown-elf-
 RISCV_GCC ?= $(RISCV_PREFIX)gcc
 RISCV_GCC_OPTS ?= -static -mcmodel=medany -fvisibility=hidden -nostdlib -nostartfiles