Add support for CYGWIN

[carlo-bramini]

This patch adds support for CYGWIN.
Hopefully, very few changes are needed.

absl/base/attributes.h
Talking about shared libraries, CYGWIN shares the same behaviour of the traditional WIN32 binaries with DLLs.
So, exactly like MinGW, the compiler doesn't complain about the weak attribute until it fails at the link step.
Here, the solution is to avoid the weak attribute, exactly like it has been already done for the existing MinGW support.

absl/base/internal/low_level_alloc.h
CYGWIN lacks support for low-level allocation of virtual memory, so here it follows again the same behaviour of WIN32.

absl/base/internal/sysinfo.cc
absl/base/internal/sysinfo.h
On CYGWIN, the function GetTID() cannot be compiled because pid_t and the return value of pthread_self() are not compatible.
pid_t is an int type, so it's 32 bit on CYGWIN.
pthread_self() returns a pthread_t which is an opaque object.
And, on CYGWIN, pthread_t is a typedef to a pointer.
When compiling for x86_64, the size of the pointer is 64bit.
So, there is a mismatch because 32bit != 64bit.
Hopefully, it is already possible to support a different pid_t into absl/base/internal/sysinfo.h, so I did the same thing that it has been done for _WIN32 and I added it to be an uintptr_t, which is good for all conditions.
However, into absl/base/internal/sysinfo.cc, the static_cast is not able to convert yet.
It can work if pthread_self() returns an int, but since pthread_t is an opaque object and it must be assumed to be like that, here the reinterpret_cast looks a better solution.

absl/base/policy_checks.h
Since CYGWIN is supported after these changes, the lines for blocking the execution of the build process are not needed anymore and they have been removed from the code.

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The library compiled fine in all conditions.
Tested build with both shared and static libraries.
Tested build with ABSL_BUILD_MONOLITHIC_SHARED_LIBS to be ON and OFF.

Sadly, I have no chance to execute bazel or Docker inside CYGWIN for running the tests because they seems to be not supported.
However, the chance that everything is working fine looks very high because CYGWIN is using code that already existed.
A direct test has been done with direct use, when building gstreamer-1.0, since abseil-cpp is a dependency.
If you think that this may be enough or something more can be done, please let me know.

I hope that you will find this patch useful.
Sincerely.

Carlo Bramini.

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