Make the assembler generate an error if there is an attempt to define a section with...

	PR 14891
	* config/obj-elf.c (obj_elf_section): Fail if the section name is
	already defined as a different symbol type.
	* testsuite/gas/elf/pr14891.s: New test source file.
	* testsuite/gas/elf/pr14891.d: New test driver.
	* testsuite/gas/elf/pr14891.s: New test expected error output.
	* testsuite/gas/elf/elf.exp: Run the new test.