[clang][riscv] add -menable-experimental-extensions

When building with clang-13 the zba riscv extension triggers a build
failure unless this switch is set. Set it for now, though it appears to
not be necessary for clang 14 and above.

TODO: add a version check for clang for a series of switches set based
on version in engine.mk.

engine.mk

@@ -220,7 +220,7 @@ ifeq ($(ARCH),arm64)
 ifeq ($(call is_warning_flag_supported,-Wasm-operand-widths),yes)
 ARCH_COMPILEFLAGS += -Wno-asm-operand-widths
 endif
-endif
+endif # arm64
 
 ifeq ($(ARCH),riscv)
 # ld.lld does not support linker relaxations yet.
@@ -240,7 +240,13 @@ ifeq ($(COMPILER_TYPE),clang)
 ARCH_COMPILEFLAGS += -fPIE -fdirect-access-external-data
 endif
 endif
+
+ifeq ($(COMPILER_TYPE),clang)
+# zba extension is reported as not supported on clang-13. Add this switch to squelch
+# the failure. TODO: gate this on a version check.
+ARCH_COMPILEFLAGS += -menable-experimental-extensions
 endif
+endif # riscv
 
 $(info PROJECT = $(PROJECT))
 $(info PLATFORM = $(PLATFORM))