[runtime] Adopt safer native compiler flags: ftrivial-auto-var-init=zero (#24085)

This will zero-initialize all automatic (stack and register based) variables.

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

@@ -31,6 +31,7 @@ CFLAGS=\
 	-Wdeprecated \
 	-Wuninitialized \
 	-fstack-protector-strong \
+	-ftrivial-auto-var-init=zero \
 	-Wformat \
 	-Wformat-security \
 	-Werror=format-security \