Split our FORTIFY implementation into libc_fortify

As requested in the bug. This also rips __memcpy_chk out of memcpy.S,
which lets us cut down on copypasta (all of the implementations look
identical).

Bug: 12231437
Test: mma on aosp_{arm,arm64,mips,x86,x86_64} internal master;
checkbuild on bullhead internal master; CtsBionicTestCases on bullhead.
No new failures.
Change-Id: I88c39ca166bacde0b692aa3063e743bb046a5d2f