[riscv][toolchain] fix clang test of -misa switch which is not present

The test inside riscv/rules.mk was assuming gcc and that the CC variable
isn't passed in from the user. This is not a very clean solution and
acts like a bandaid over the problem. Added some todos for a potential
solution.

arch/arm64/rules.mk

@@ -111,6 +111,7 @@ ARCH_COMPILEFLAGS_FLOAT :=
 
 ARCH_LDFLAGS += -z max-page-size=$(ARM64_PAGE_SIZE)
 
+# Note: assumes the use of gcc and the user is not overriding CC which is set later in engine.mk
 LIBGCC := $(shell $(TOOLCHAIN_PREFIX)gcc $(GLOBAL_COMPILEFLAGS) $(ARCH_COMPILEFLAGS) -print-libgcc-file-name)
 
 # make sure some bits were set up

arch/riscv/rules.mk

@@ -183,7 +183,12 @@ endif
 
 # test to see if -misa-spec=2.2 is a valid switch.
 # misa-spec is added to make sure the compiler picks up the zicsr extension by default.
+# If CC is being overridden by the user, use that instead of the toolchain gcc.
+ifdef CC
+MISA_SPEC := $(shell $(CC) $(ARCH_COMPILEFLAGS) -misa-spec=2.2 -E - < /dev/null > /dev/null 2>1 && echo supported)
+else
 MISA_SPEC := $(shell $(TOOLCHAIN_PREFIX)gcc $(ARCH_COMPILEFLAGS) -misa-spec=2.2 -E - < /dev/null > /dev/null 2>1 && echo supported)
+endif
 $(info MISA_SPEC = $(MISA_SPEC))
 ifeq ($(MISA_SPEC),supported)
 ARCH_COMPILEFLAGS += -misa-spec=2.2

engine.mk

@@ -202,6 +202,10 @@ $(info TOOLCHAIN_PREFIX = $(TOOLCHAIN_PREFIX))
 list-toolchain:
 else
 
+# TODO: rethink how this works and perhaps have the arch rules.mk set up CC, OBJCOPY, etc
+# so that they can be a bit more flexible in how they are set up.
+# Also consider using ARCH_CC, etc to disambiguate between host and target toolchains.
+
 # default to no ccache
 CCACHE ?=
 CC ?= $(CCACHE) $(TOOLCHAIN_PREFIX)gcc