[zbin] Allow for constructing compressed dynamic relocation records

[zbin] Allow for constructing compressed dynamic relocation records

Define a new "ZREL" compressor information block, describing a block
of Elf_Rel or Elf_Rela runtime relocations to be converted to an
iPXE-specific compressed relocation format.

The compressed relocation format is based loosely on the Elf_Relr
bitmap+offset format, with some optimisations for use in iPXE.  In
particular:

  - a relative "skip" value is used instead of an absolute offset

  - the width of the skip value is reduced to 19 bits (when present)

  - an explicit skip value of zero is used to terminate the list

  - unaligned relocations are prohibited

The layout of bits within the compressed relocation record is also
adjusted to make assembly code implementations simpler: the skip flag
bit is placed in the MSB so that it can be tested using "bltz" or
similar instructions, and the skip value is placed above the
relocation flag bits so that a typical shifting implementation will
naturally end up with a zero value in its accumulator if and only if
the record was a terminator.

Signed-off-by: Michael Brown <mcb30@ipxe.org>