lua: remove unsupported flags for llvm

Prevents error:

  clang: error: unknown argument: '-fstack-usage'
  clang: error: unknown argument: '-fconserve-stack'

The flags are here to print debug information and try to optimize stack but are
not required.

pkg/lua/Makefile.lua

 SRC := $(filter-out loadlib.c lua.c luac.c,$(wildcard *.c))
 
-CFLAGS += -fstack-usage -fconserve-stack \
-          -DLUA_MAXCAPTURES=16 \
-          -DL_MAXLENNUM=50
+ifneq (llvm, $(TOOLCHAIN))
+  CFLAGS += -fstack-usage -fconserve-stack
+endif
+
+CFLAGS += -DLUA_MAXCAPTURES=16 -DL_MAXLENNUM=50
+
 #    Enable these options to debug stack usage
 #          -Wstack-usage=128 -Wno-error=stack-usage=128