build: clean up .dwo files

When building with -gsplit-dwarf, the generated dwo files are left behind
even when you `make clean`.  Fix that up.

BUG=chromium:579384
TEST=`./configure CXXFLAGS='-O -gsplit-dwarf' && make && make clean` removes dwo files now
R=mark@chromium.org

Review URL: https://codereview.chromium.org/1633893002 .

.gitignore

@@ -31,6 +31,7 @@
 
 # Ignore common compiled artifacts.
 *~
+*.dwo
 *.o
 lib*.a
 /breakpad.pc

Makefile.am

@@ -1304,3 +1304,6 @@ EXTRA_DIST = \
 	src/tools/windows/dump_syms/testdata/dump_syms_regtest.sym \
 	src/tools/windows/symupload/symupload.cc \
 	src/tools/windows/symupload/symupload.vcproj
+
+mostlyclean-local:
+	-find src -name '*.dwo' -exec rm -f {} +

Makefile.in

@@ -7728,7 +7728,8 @@ maintainer-clean-am: distclean-am maintainer-clean-generic
 
 mostlyclean: mostlyclean-am
 
-mostlyclean-am: mostlyclean-compile mostlyclean-generic
+mostlyclean-am: mostlyclean-compile mostlyclean-generic \
+	mostlyclean-local
 
 pdf: pdf-am
 
@@ -7769,8 +7770,8 @@ uninstall-am: uninstall-binPROGRAMS uninstall-dist_docDATA \
 	install-pdf-am install-pkgconfigDATA install-ps install-ps-am \
 	install-strip installcheck installcheck-am installdirs \
 	maintainer-clean maintainer-clean-generic mostlyclean \
-	mostlyclean-compile mostlyclean-generic pdf pdf-am ps ps-am \
-	recheck tags tags-am uninstall uninstall-am \
+	mostlyclean-compile mostlyclean-generic mostlyclean-local pdf \
+	pdf-am ps ps-am recheck tags tags-am uninstall uninstall-am \
 	uninstall-binPROGRAMS uninstall-dist_docDATA \
 	uninstall-includecHEADERS uninstall-includeclHEADERS \
 	uninstall-includeclcHEADERS uninstall-includecldwcHEADERS \
@@ -7782,6 +7783,9 @@ uninstall-am: uninstall-binPROGRAMS uninstall-dist_docDATA \
 .PRECIOUS: Makefile
 
 
+mostlyclean-local:
+	-find src -name '*.dwo' -exec rm -f {} +
+
 # Tell versions [3.59,3.63) of GNU make to not export all variables.
 # Otherwise a system limit (for SysV at least) may be exceeded.
 .NOEXPORT: