Libbacktrace: Fix the use of newline in sed replacement

On non-ELF targets, the Makefile needs a newline inside the sed REPLACE
string. The way it is currently done fails with GNU Make < 4, but GCC
only requires "GNU make version 3.80 (or later)".

The portable solution is given in the autoconf manual:
https://www.gnu.org/software/autoconf/manual/autoconf-2.69/html_node/Newlines-in-Make-Rules.html

libbacktrace/ChangeLog:

	PR libbacktrace/103822

	* Makefile.am: Fix newline.
	* Makefile.in: Regenerate.