main/ruby: switch to libucontext

- Clarify license
- Cleanup
- FTBS - use libucontext instead of buildin coroutines asm:
/usr/lib/gcc/s390x-alpine-linux-musl/9.3.0/../../../../s390x-alpine-linux-musl/bin/ld: cont.o: in function `coroutine_initialize_main':
cont.c:(.text+0x260): undefined reference to `getcontext'
/usr/lib/gcc/s390x-alpine-linux-musl/9.3.0/../../../../s390x-alpine-linux-musl/bin/ld: cont.o: in function `fiber_setcontext':
cont.c:(.text+0xeac): undefined reference to `swapcontext'
/usr/lib/gcc/s390x-alpine-linux-musl/9.3.0/../../../../s390x-alpine-linux-musl/bin/ld: cont.o: in function `fiber_switch':
cont.c:(.text+0x1962): undefined reference to `makecontext'
collect2: error: ld returned 1 exit status
7 jobs for !7082 with main/ruby in 9 minutes and 35 seconds
detached
Status Job ID Name Coverage
  Lint
failed #98798
x86_64 docker-alpine allowed to fail
lint

00:00:20

 
  Build
passed #98803
aarch64 ci-build docker-alpine
build-aarch64

00:06:18

passed #98804
armv7 ci-build docker-alpine
build-armv7

00:06:36

passed #98802
ppc64le ci-build docker-alpine
build-ppc64le

00:09:14

passed #98801
s390x ci-build docker-alpine
build-s390x

00:03:03

passed #98800
x86 ci-build docker-alpine
build-x86

00:04:46

passed #98799
x86_64 ci-build docker-alpine
build-x86_64

00:03:47

 
Name Stage Failure
failed
lint Lint
No job log