arm: define PIC based on __PIC__ or __pic__ if not defined

Fixes #149.
12 jobs for master in 5 minutes and 45 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #234911
amd64 debian
build-debian

00:00:31

passed #234915
debian aarch64
build-debian-aarch64

00:01:40

passed #234916
debian clang5 aarch64
build-debian-aarch64-clang-5

00:01:18

passed #234912
amd64 debian
build-debian-static

00:00:31

passed #234918
debian aarch64
build-debian-werror

00:00:33

passed #234917
macos
build-macos

00:00:25

passed #234913
win32
build-win32

00:00:29

passed #234914
win64
build-win64

00:00:34

 
  Test
passed #234919
amd64 debian
test-debian

00:00:39

passed #234920
amd64 debian
test-debian-asan

00:01:56

passed #234921
aarch64 debian
test-debian-msan

00:01:13

passed #234922
amd64 debian
test-debian-ubsan

00:00:59