[BPF] Add BTF 64bit enum value support

Current BTF only supports 32-bit value. For example,
  enum T { VAL = 0xffffFFFF00000008 };
the generated BTF looks like
        .long   16                              # BTF_KIND_ENUM(id = 4)
        .long   100663297                       # 0x6000001
        .long   8
        .long   18
        .long   8
The encoded value is 8 which equals to (uint32_t)0xffffFFFF00000008
and this is incorrect.

This patch introduced BTF_KIND_ENUM64 which permits to encode
64-bit value. The format for each enumerator looks like:
        .long   name_offset
        .long   (uint32_t)value # lower-32 bit value
        .long   value >> 32     # high-32 bit value

We use two 32-bit values to represent a 64-bit value as current
BTF type subsection has 4-byte alignment and gaps are not permitted
in the subsection.

This patch also added support for kflag (the bit 31 of CommonType.Info)
such that kflag = 1 implies the value is signed and kflag = 0
implies the value is unsigned. The kernel UAPI enumerator definition is
  struct btf_enum {
        __u32   name_off;
        __s32   val;
  };
so kflag = 0 with unsigned value provides backward compatability.

With this patch, for
  enum T { VAL = 0xffffFFFF00000008 };
the generated BTF looks like
        .long   16                              # BTF_KIND_ENUM64(id = 4)
        .long   3187671053                      # 0x13000001
        .long   8
        .long   18
        .long   8                               # 0x8
        .long   4294967295                      # 0xffffffff
and the enumerator value and signedness are encoded correctly.

Differential Revision: https://reviews.llvm.org/D124641