Enable Alpine build (#4579)
Enable Alpine build (#4579)

Using Linux distro detection part of CoreCLR's init-tools.sh here.

The following DockerFile is used to build on Alpine Linue:

```dockerfile
FROM alpine:3.6
RUN cat /etc/*-release

RUN apk update
RUN apk add --no-cache \
        autoconf bash build-base clang clang-dev \
        cmake curl-dev gcc gettext-dev git icu-dev libtool \
        linux-headers libunwind-dev llvm make openssl zlib-dev \
        util-linux-dev

# note that the testing URL will be changed once lldb-dev is moved to
# aports' (more prominent) "main" repo.
# See https://github.com/alpinelinux/aports/pull/2342#issuecomment-330894890
RUN apk add --no-cache \
       -X https://dl-cdn.alpinelinux.org/alpine/edge/testing \
        lldb-dev

RUN git clone https://github.com/dotnet/corert -b master --single-branch \
    && cd corert && ./build.sh
```
gregkalapos/corert,shrah/corert,shrah/corert,shrah/corert,gregkalapos/corert,gregkalapos/corert,gregkalapos/corert,shrah/corert