msp430.c (msp430_attr): Warn when the critical and interrupt function attributes are used together.

2018-12-29  Jozef Lawrynowicz  <jozef.l@mittosystems.com>

	gcc/ChangeLog:

	* config/msp430/msp430.c (msp430_attr): Warn when the critical and
	interrupt function attributes are used together.
	* gcc/doc/extend.texi: Update documentation on the critical attribute.

	gcc/testsuite/ChangeLog:

	* gcc.target/msp430/critical-interrupt.c: New test.

From-SVN: r267467