MSP430: Fix alignment of __*_array_start symbols in default linker script

MSP430: Fix alignment of __*_array_start symbols in default linker script

__{preinit,init,fini}_array_start symbols must be word aligned in
linker scripts. If the section preceding the __*_array_start symbol
has an odd size, then a NULL byte will be present between the start
symbol and the .*_array section itself, when the section gets
automatically word-aligned.

This results in a branch to an invalid address when the CRT startup
code tries to run through the functions listed in the array sections.

Some MSP430 linker scripts do not align the __*_array start symbols, so
this added warning will catch that problem and help the user avoid
the potential incorrect execution of the program.

ld/ChangeLog:

	* emultempl/msp430.em (input_section_exists): New.
	(check_array_section_alignment): New.
	(gld${EMULATION_NAME}_finish): New.
	* scripttempl/elf32msp430.sc: Add ALIGN directives before the
	definition of __*_array_start symbols.
	* testsuite/ld-msp430-elf/finiarray-warn.ld: New test.
	* testsuite/ld-msp430-elf/finiarray-warn.r: New test.
	* testsuite/ld-msp430-elf/initarray-nowarn.ld: New test.
	* testsuite/ld-msp430-elf/initarray-warn.ld: New test.
	* testsuite/ld-msp430-elf/initarray-warn.r: New test.
	* testsuite/ld-msp430-elf/initarray.s: New test.
	* testsuite/ld-msp430-elf/msp430-elf.exp: Run new tests.
	* testsuite/ld-msp430-elf/preinitarray-warn.ld: New test.
	* testsuite/ld-msp430-elf/preinitarray-warn.r: New test.