x86: add endbr32 and endbr64 instructions

Issue endbr instructions at the start of generated functions to
make IBT (indirect branch tracking) work.

Resolves #17
4 jobs for master in 1 minute and 7 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Build
passed #442605
debian sid

00:00:42

passed #442608
docker windows 1607
msys2

00:01:04

passed #442606
docker windows 1607
vs2017 amd64

00:01:04

passed #442607
docker windows 1607
vs2017 x86

00:01:07