Add a comment explaining why stack_capture is empty for EABI and clang.

While here add a comment pointing out that, while r11 is not the frame
pointer on EABI as there is no frame pointer, it's value is unused so is
safe.
Re-introduce another part of r249367. This commit fixes a register leak in
dt_cg_ptrsize() and generally cleans up some of the error handling around
register allocation.

This change corresponds to part of illumos-gate commit e5803b76927480:
  3025 register leak in D code generation

Reviewed by:	pfg
Obtained from:	illumos
MFC after:	1 month