RISC-V/GAS: Improve handling of invalid relocs
    
    TLS relocs against constants previously segfaulted, and illegal
    symbol subtractions were silently ignored.
    
    The previous behavior was to segfault.
    
    gas/ChangeLog
    
    2017-01-09  Andrew Waterman <andrew@sifive.com>
    
    	* config/tc-riscv.c (md_apply_fix): Report TLS relocations against
    	constants.  Report disallowed symbol subtractions.