stm32/Makefile: Fix C++ linker flags when toolchain has spaces in path.
stm32/Makefile: Fix C++ linker flags when toolchain has spaces in path.

The GNU Make dir command uses spaces as item separator so it does not
work for e.g building the STM32 port on Cygwin with a default Arm
installation in "c:/program files (x86)/GNU Arm Embedded Toolchain".
Fix by using POSIX dirname on a quoted path instead.

pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython