arch: arm: armv6-m: fix LTO build

This imports changes from armv7-m.

arch/arm/src/armv6-m/Toolchain.defs

@@ -100,6 +100,9 @@ ifeq ($(CONFIG_LTO_THIN),y)
   ARCHOPTIMIZATION += -flto=thin
 else ifeq ($(CONFIG_LTO_FULL),y)
   ARCHOPTIMIZATION += -flto
+  ifeq ($(CONFIG_ARMV6M_TOOLCHAIN),GNU_EABI)
+    ARCHOPTIMIZATION += -fuse-linker-plugin
+  endif
 endif
 
 # NuttX buildroot under Linux or Cygwin
@@ -143,12 +146,21 @@ else
   CC      = $(CROSSDEV)gcc
   CXX     = $(CROSSDEV)g++
   CPP     = $(CROSSDEV)gcc -E -P -x c
-  LD      = $(CROSSDEV)ld
   STRIP   = $(CROSSDEV)strip --strip-unneeded
-  AR      = $(CROSSDEV)ar rcs
-  NM      = $(CROSSDEV)nm
   OBJCOPY = $(CROSSDEV)objcopy
   OBJDUMP = $(CROSSDEV)objdump
+  LD      = $(CROSSDEV)ld
+  AR      = $(CROSSDEV)ar rcs
+  NM      = $(CROSSDEV)nm
+
+  ifeq ($(CONFIG_LTO_FULL),y)
+    ifeq ($(CONFIG_ARMV6M_TOOLCHAIN),GNU_EABI)
+      LD := $(CROSSDEV)gcc
+      AR := $(CROSSDEV)gcc-ar rcs
+      NM := $(CROSSDEV)gcc-nm
+      ARCHOPTIMIZATION += -fno-builtin
+    endif
+  endif
 endif
 
 # Architecture flags
@@ -171,6 +183,8 @@ ifneq ($(CONFIG_CXX_RTTI),y)
   ARCHCXXFLAGS += -fno-rtti
 endif
 
+LDFLAGS += -nostdlib
+
 # Optimization of unused sections
 
 ifeq ($(CONFIG_DEBUG_OPT_UNUSED_SECTIONS),y)