Add a pair of MIPS16 branch tests to verify correct R_MIPS16_PC16_S1
relocation generation for cross-section references in a single source.
This complements commit c9775dde ("MIPS16: Add R_MIPS16_PC16_S1
branch relocation support").

	gas/
	* testsuite/gas/mips/mips16-branch-reloc-4.d: New test.
	* testsuite/gas/mips/mips16-branch-reloc-5.d: New test.
	* testsuite/gas/mips/mips16-branch-reloc-4.s: New test source.
	* testsuite/gas/mips/mips16-branch-reloc-5.s: New test source.
	* testsuite/gas/mips/mips.exp: Run the new tests.