Add gas support for half-precision floating point

* Add the Xhalf RISC-V extension flag (use -march=RV...Xhalf to use
  half-precision floating point operations).
* Add sensible combinations of half-precision floating-point opcodes to
  gas and gcc (through the riscv-opcodes repository).
Add missing fcvt opcodes
fix slli/slliw encoding bug
Merge branch 'master' of github.com:ucb-bar/riscv-gcc into confprec
Add vsetprec instruction
Add missing break and continue statements

* With thanks to tommythorn
Merge branch 'confprec'

Conflicts:
	binutils-2.21.1/gas/config/tc-riscv.c
	binutils-2.21.1/include/opcode/riscv-opc.h
	binutils-2.21.1/opcodes/riscv-opc.c