cpu/cortexm_common: enable LTO by default

LTO support in GCC should be pretty mature by now, to a point where
Fedora enables it by default for all packages[0].

Let's follow suit by enabling LTO for our Cortex-M target.
This is the most mature target and is well-tested regularly.
So if any issues are caused by this, they should pop up fairly quick.

[0] https://lists.fedoraproject.org/archives/list/devel@lists.fedoraproject.org/thread/CQEI3UGK5HSHZ7JHE7NRTQHBWQYWA5HM/

makefiles/arch/cortexm.inc.mk

@@ -8,6 +8,9 @@ TARGET_ARCH ?= arm-none-eabi
 # define build specific options
 CFLAGS_CPU   = -mcpu=$(MCPU) -mlittle-endian -mthumb $(CFLAGS_FPU)
 
+# enable LTO by default
+LTO ?= 1
+
 ifneq (llvm,$(TOOLCHAIN))
   # Clang (observed with v3.7) does not understand  -mno-thumb-interwork, only add if
   # not building with LLVM