[arch] make sure the arch rules file sets TOOLCHAIN_PREFIX, make sure riscv picks up the alt selector

ARCH_riscv*_TOOLCHAIN_PREFIX should override TOOLCHAIN_PREFIX like it does on some other arches.
While was at it clean up the rules.mk file a bit.

arch/riscv/rules.mk

@@ -116,23 +116,24 @@ GLOBAL_DEFINES += ROMBASE=$(ROMBASE)
 GLOBAL_DEFINES += MEMBASE=$(MEMBASE)
 GLOBAL_DEFINES += MEMSIZE=$(MEMSIZE)
 
+# if ARCH_riscv{32|64}_TOOLCHAIN_PREFIX is set use it as an override
+# for toolchain prefix.
+ifdef ARCH_$(ARCH)$(SUBARCH)_TOOLCHAIN_PREFIX
+TOOLCHAIN_PREFIX := $(ARCH_$(ARCH)$(SUBARCH)_TOOLCHAIN_PREFIX)
+endif
+
+# default toolchain is riscv{32|64}-elf-. assume its in the path.
+ifndef TOOLCHAIN_PREFIX
+TOOLCHAIN_PREFIX := riscv$(SUBARCH)-elf-
+endif
+
 # based on 32 or 64 bitness, select the right toolchain and some
 # compiler codegen flags
 ifeq ($(SUBARCH),32)
-
-ifndef TOOLCHAIN_PREFIX
-TOOLCHAIN_PREFIX := riscv32-elf-
-endif
 ARCH_COMPILEFLAGS := -march=rv32imac -mabi=ilp32
-
 else ifeq ($(SUBARCH),64)
-
-ifndef TOOLCHAIN_PREFIX
-TOOLCHAIN_PREFIX := riscv64-elf-
-endif
 ARCH_COMPILEFLAGS := -march=rv64imac -mabi=lp64 -mcmodel=medany
 GLOBAL_DEFINES += IS_64BIT=1
-
 else
 $(error SUBARCH not set or set to something unknown)
 endif
@@ -142,11 +143,11 @@ endif
 ifeq (true,$(call TOBOOL,$(ARCH_RISCV_EMBEDDED)))
 ARCH_OPTFLAGS ?= -Os
 GLOBAL_DEFINES += ARCH_RISCV_EMBEDDED=1
+WITH_LINKER_GC ?= 1
 else
 ARCH_OPTFLAGS ?= -O2
-endif
-
 WITH_LINKER_GC ?= 0
+endif
 
 LIBGCC := $(shell $(TOOLCHAIN_PREFIX)gcc $(GLOBAL_COMPILEFLAGS) $(ARCH_COMPILEFLAGS) $(GLOBAL_CFLAGS) -print-libgcc-file-name)
 $(info LIBGCC = $(LIBGCC))

engine.mk

@@ -148,11 +148,15 @@ ifndef ARCH
 $(error couldn't find arch or platform doesn't define arch)
 endif
 include arch/$(ARCH)/rules.mk
+ifndef TOOLCHAIN_PREFIX
+$(error TOOLCHAIN_PREFIX not set in the arch rules.mk)
+endif
 
 $(info PROJECT = $(PROJECT))
 $(info PLATFORM = $(PLATFORM))
 $(info TARGET = $(TARGET))
 $(info ARCH = $(ARCH))
+$(info TOOLCHAIN_PREFIX = $(TOOLCHAIN_PREFIX))
 
 # include the top level module that includes basic always-there modules
 include top/rules.mk
@@ -232,6 +236,7 @@ GLOBAL_DEFINES += ARCH_CFLAGS=\"$(subst $(SPACE),_,$(ARCH_CFLAGS))\"
 GLOBAL_DEFINES += ARCH_CPPFLAGS=\"$(subst $(SPACE),_,$(ARCH_CPPFLAGS))\"
 GLOBAL_DEFINES += ARCH_ASMFLAGS=\"$(subst $(SPACE),_,$(ARCH_ASMFLAGS))\"
 GLOBAL_DEFINES += ARCH_LDFLAGS=\"$(subst $(SPACE),_,$(ARCH_LDFLAGS))\"
+GLOBAL_DEFINES += TOOLCHAIN_PREFIX=\"$(subst $(SPACE),_,$(TOOLCHAIN_PREFIX))\"
 
 ifneq ($(OBJS),)
 $(warning OBJS=$(OBJS))