Fix compiler warning in psp_test.c when compiled with ARM compiler.

Fix compiler warning in psp_test.c when compiled with ARM compiler.
Add portYIELD_FROM_ISR() macros to Cortex-M ports.  The new macro just calls the exiting portEND_SWITCHING_ISR() macro.
Remove code from the MSVC port layer that was left over from a previous implementation and become obsolete.

git-svn-id: https://svn.code.sf.net/p/freertos/code/trunk@1898 1d2547de-c912-0410-9cb9-b8ca96c0e9e2