Fix build of stand/ when building world with ASAN

The userboot/test  program links against the default userspace libraries
(e.g. shared libgcc_s.so) that will be instrumented if WITH_ASAN is set.
All other programs link against libsa instead of libc and therefore can't
use the sanitizer runtime library. To fix the stand/ build with
sanitizers, we disable MK_ASAN/MK_UBSAN if -nostdlib is found in the
LDFLAGS (i.e. we are using libsa instead of libc).

Reviewed By:	imp, tsoome
Differential Revision: https://reviews.freebsd.org/D31047

stand/defs.mk

@@ -24,6 +24,12 @@ INTERNALLIB=
 # enough to make that hassle worth chasing.
 _CPUCFLAGS=
 
+.if ${LDFLAGS:M-nostdlib}
+# Sanitizers won't work unless we link against libc (e.g. in userboot/test).
+MK_ASAN:=	no
+MK_UBSAN:=	no
+.endif
+
 .include <src.opts.mk>
 .include <bsd.linker.mk>
 

stand/userboot/userboot/Makefile

@@ -6,6 +6,9 @@ LOADER_CD9660_SUPPORT?=	no
 LOADER_EXT2FS_SUPPORT?=	no
 PIC=yes
 
+# Note: -nostdlib needs to be added to LDFLAGS before including defs.mk
+LDFLAGS+=	-nostdlib -Wl,-Bsymbolic
+
 .include <bsd.init.mk>
 
 SHLIB_NAME=	userboot_${LOADER_INTERP}.so
@@ -39,8 +42,6 @@ CFLAGS.main.c+=	-I${SYSDIR}/contrib/openzfs/include/os/freebsd/zfs
 CFLAGS.gfx_fb.c+= -I$(SRCTOP)/sys/teken -I${SRCTOP}/contrib/pnglite
 CWARNFLAGS.main.c += -Wno-implicit-function-declaration
 
-LDFLAGS+=	-nostdlib -Wl,-Bsymbolic
-
 NEWVERSWHAT=	"User boot ${LOADER_INTERP}" ${MACHINE_CPUARCH}
 VERSION_FILE=	${.CURDIR}/../userboot/version