md/define_c_enum: support value assignation

md/define_c_enum: support value assignation

Currently, the enums from define_c_enum and define_enum can only
has values one by one from 0.

In fact we can support the behaviour just like C, aka like
  (define_enum "mips_isa" [(mips1 1) mips2 (mips32 32) mips32r2]),
then we can get
  enum mips_isa {
    MIPS_ISA_MIPS1 = 1,
    MIPS_ISA_MIPS2 = 2,
    MIPS_ISA_MIPS32 = 32,
    MIPS_ISA_MIPS32R2 = 33
  };

gcc/ChangeLog:
	* read-md.c (md_reader::handle_enum): support value assignation.
	* doc/md.texi: record define_c_enum value assignation support.