Fix build with clang by using llvm-ar

When building with clang and RV32F enabled, linking fails with
undefined references to SoftFloat functions. For example:

$ make CC=clang-18

/usr/bin/ld: /tmp/lto-llvm-28abb9.o: in function `do_fmadds':
emulate.c:(.text.do_fmadds+0x3c): undefined reference to `f32_mulAdd'
/usr/bin/ld: /tmp/lto-llvm-28abb9.o: in function `do_fadds':
emulate.c:(.text.do_fadds+0x31): undefined reference to `f32_add'

This happens because GNU ar cannot correctly archive objects compiled
with clang. Switch to llvm-ar when CC is clang, ensuring the SoftFloat
library is archived correctly and the build succeeds.

Author: visitorckw

Makefile

@@ -115,6 +115,9 @@ ENABLE_EXT_F ?= 1
 $(call set-feature, EXT_F)
 ifeq ($(call has, EXT_F), 1)
 AR := ar
+ifeq ("$(CC_IS_CLANG)", "1")
+AR = llvm-ar
+endif
 ifeq ("$(CC_IS_EMCC)", "1")
 AR = emar
 endif