build: make: use -Wl,-z,separate-code w/ -Wl,--execute-only

Also, prefer
-Wl,--execute-only
rather than
-Wl,-execute-only

Bug: 139945549
Bug: 146144180
Test: m
Change-Id: I4a07978c7a84a0a7e841b9e059305b0f443d0e95
Suggested-by: Ryan Prichard <rprichard@google.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>