ALPHA_R_OP_STORE

ALPHA_R_OP_STORE

In commit db4ab410dec3 I rewrote OP_STORE handling to support writing
near the end of a section.  The rewrite had some bugs, fixed in commit
3e02c4891dcb.  However I wasn't entirely happy with the code writing
the bitfield:
- it doesn't support 64-bit fields with a bit offset,
- the code is duplicated and inelegant,
- the stack ought to be popped whenever seeing one of these relocs,
  even if the reloc can't be applied.
This patch fixes all of the above.

In addition, it is clear from the OP_STORE description in the ABI that
a 64-bit field is encoded as 0 in r_size, so I've decoded that in
alpha_ecoff_swap_reloc_in.  The aborts there are not appropriate as
they can be triggered by user input (fuzzed object files).  Also,
stack underflow wasn't checked in alpha_relocate_section.

	* coff-alpha.c (alpha_ecoff_swap_reloc_in): Replace aborts
	with asserts.  Decode ALPHA_R_OP_STORE r_size of zero.
	(write_bit_field): New function.
	(alpha_ecoff_get_relocated_section_contents): Use it.
	(alpha_relocate_section): Here too.  Catch stack underflow.