Enable gcovr --exclude-unreachable-branches and --exclude-throw-branches flags

These are mostly useful for C++ code, i.e. ext-intl. See:
https://gcovr.com/en/stable/faq.html#why-does-c-code-have-so-many-uncovered-branches

Closes phpGH-21655

Author: iluuu1994

build/Makefile.gcov

@@ -56,14 +56,14 @@ gcovr-html:
 	@echo "Generating gcovr HTML"
 	@rm -rf gcovr_html/
 	@mkdir gcovr_html
-	gcovr -sr . -o gcovr_html/index.html --html --html-details \
+	gcovr -sr . -o gcovr_html/index.html --html --html-details --exclude-unreachable-branches --exclude-throw-branches \
 		$(foreach pattern, $(GCOVR_EXCLUDE_LINES_BY_PATTERNS), --exclude-lines-by-pattern $(pattern)) \
 		$(foreach lib, $(GCOVR_EXCLUDES), -e $(lib))
 
 gcovr-xml:
 	@echo "Generating gcovr XML"
 	@rm -f gcovr.xml
-	gcovr -sr . -o gcovr.xml --xml \
+	gcovr -sr . -o gcovr.xml --xml --exclude-unreachable-branches --exclude-throw-branches \
 		$(foreach pattern, $(GCOVR_EXCLUDE_LINES_BY_PATTERNS), --exclude-lines-by-pattern $(pattern)) \
 		$(foreach lib, $(GCOVR_EXCLUDES), -e $(lib))