selftests/nolibc: add support for LLVM= parameter

Makefile.include can modify CC and CFLAGS for usage with clang.
Make use of it.

Makefile.include is currently used to handle the O= variable.
This is incompatible with the LLVM= handling as for O= it has to be
included as early as possible, while for LLVM= it needs to be included
after CFLAGS are set up.

To avoid this incompatibility, switch the O= handling to custom logic.
Reviewed-by: Shuah Khan <skhan@linuxfoundation.org>
Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/r/20240807-nolibc-llvm-v2-10-c20f2f5fc7c2@weissschuh.netSigned-off-by: Thomas Weißschuh <linux@weissschuh.net>

tools/testing/selftests/nolibc/Makefile

@@ -5,7 +5,6 @@ ifeq ($(srctree),)
 srctree := $(patsubst %/tools/testing/selftests/,%,$(dir $(CURDIR)))
 endif
 
-include $(srctree)/tools/scripts/Makefile.include
 include $(srctree)/tools/scripts/utilities.mak
 # We need this for the "cc-option" macro.
 include $(srctree)/tools/build/Build.include
@@ -14,6 +13,9 @@ ifneq ($(O),)
 ifneq ($(call is-absolute,$(O)),y)
 $(error Only absolute O= parameters are supported)
 endif
+objtree := $(O)
+else
+objtree ?= $(srctree)
 endif
 
 ifeq ($(ARCH),)
@@ -21,8 +23,6 @@ include $(srctree)/scripts/subarch.include
 ARCH = $(SUBARCH)
 endif
 
-objtree ?= $(srctree)
-
 # XARCH extends the kernel's ARCH with a few variants of the same
 # architecture that only differ by the configuration, the toolchain
 # and the Qemu program used. It is copied as-is into ARCH except for
@@ -155,6 +155,9 @@ CFLAGS  ?= -Os -fno-ident -fno-asynchronous-unwind-tables -std=c89 -W -Wall -Wex
 		$(CFLAGS_$(XARCH)) $(CFLAGS_STACKPROTECTOR) $(CFLAGS_EXTRA)
 LDFLAGS :=
 
+# Modify CFLAGS based on LLVM=
+include $(srctree)/tools/scripts/Makefile.include
+
 REPORT  ?= awk '/\[OK\][\r]*$$/{p++} /\[FAIL\][\r]*$$/{if (!f) printf("\n"); f++; print;} /\[SKIPPED\][\r]*$$/{s++} \
 		END{ printf("\n%3d test(s): %3d passed, %3d skipped, %3d failed => status: ", p+s+f, p, s, f); \
 		if (f || !p) printf("failure\n"); else if (s) printf("warning\n"); else printf("success\n");; \