[libbacktrace] Fix and simplify xcoff_%.c pattern rule

[libbacktrace] Fix and simplify xcoff_%.c pattern rule

When generating xcoff_%.c, the last command is a sed command.  In case of a
sed failure, this will leave an incomplete file, which will appear as up to
date to make, so consequently it will not be regenerated.  Fix this by
sedding into a temporary file instead.

Also, use $< to access the prerequisite xcoff.c, instead of spelling out the
file name once more.

2019-01-28  Tom de Vries  <tdevries@suse.de>

	* Makefile.am (xcoff_%.c): Generate sed result into temporary file.
	Use $< to access prerequisite.
	* Makefile.in: Regenerate.

From-SVN: r268344