aarch64-pe can't fill 16 bytes in section .text

Without commit b66e671854, this:
 .p2align 4
 nop
 .p2align 3
 nop
results in an error when coff_frob_section attempts to pad out the
section to a 16-byte boundary.  Due to miscalculating the pad pattern
repeat count, write.c:write_contents attempts to shove 16 bytes of
padding into the remaining 4 bytes of the .text section.

	* config/obj-coff.c (coff_frob_section): Correct fill count.
	Don't pad after errors.