[runtime] Adopt safer native compiler flags: report numerous other warnings (#24093)

None of these warnings trigger in our code base at the moment.

References:

* https://developer.apple.com/documentation/xcode/enabling-enhanced-security-for-your-app
* https://releases.llvm.org/8.0.0/tools/clang/docs/ReleaseNotes.html#major-new-features (for -ftrivial-auto-var-init=zero)

Contributes towards #23023.

Author: rolfbjarne

mk/rules.mk

@@ -25,6 +25,9 @@ macos_DEFINES=-DMONOMAC
 OBJC_CFLAGS=-ObjC++ -std=c++14 -fno-exceptions -fno-objc-msgsend-selector-stubs -fobjc-abi-version=2 -fobjc-legacy-dispatch
 CFLAGS=\
 	-Wall \
+	-Wextra \
+	-Wno-unused-parameter \
+	-Wno-unused-but-set-parameter \
 	-fms-extensions \
 	-Werror \
 	-Wconversion \
@@ -43,7 +46,15 @@ CFLAGS=\
 	-Wsemicolon-before-method-body \
 	-Wsign-compare \
 	-Wshadow \
+	-Wempty-body \
+	-Wbuiltin-memcpy-chk-size \
 	-Wformat-nonliteral \
+	-Warray-bounds \
+	-Warray-bounds-pointer-arithmetic \
+	-Wsuspicious-memaccess \
+	-Wsizeof-array-div \
+	-Wsizeof-pointer-div \
+	-Wreturn-stack-address \
 	-g \
 	-I.
 SWIFTFLAGS=-g -emit-library