objtool is using the headers provided by kernel-libc-headers, which are kernel version 5.10, so
they use __always_inline instead of inline, and musl doesn't define __always_inline (glibc does)

diff --git a/tools/objtool/Makefile b/tools/objtool/Makefile
index 92ce4fc..d74b931 100644
--- a/tools/objtool/Makefile
+++ b/tools/objtool/Makefile
@@ -30,7 +30,7 @@ INCLUDES := -I$(srctree)/tools/include \
 	    -I$(srctree)/tools/objtool/include \
 	    -I$(srctree)/tools/objtool/arch/$(SRCARCH)/include
 WARNINGS := $(EXTRA_WARNINGS) -Wno-switch-default -Wno-switch-enum -Wno-packed -Wno-nested-externs
-CFLAGS   := -Werror $(WARNINGS) $(KBUILD_HOSTCFLAGS) -g $(INCLUDES) $(LIBELF_FLAGS)
+CFLAGS   := -Werror $(WARNINGS) $(KBUILD_HOSTCFLAGS) -g $(INCLUDES) $(LIBELF_FLAGS) -D__always_inline=inline
 LDFLAGS  += $(LIBELF_LIBS) $(LIBSUBCMD) $(KBUILD_HOSTLDFLAGS)
 
 # Allow old libelf to be used:
