Turn on -Werror=implicit-function-declaration for x86-64.

Implicit function declarations are much more dangerous on LP64 because
sizeof(int) != sizeof(void*), so any function that returns a pointer will
lose its top bits, leading to relatively hard to debug crashes.

Change-Id: Ib2570326dde1b1cde09c63c9b4c973d741528ee2
1 file changed