Support Mac package/DMG Notarization

macosx8664 --> macos

We're building for both kinds of CPUs now.

Move official releases to GitHub and packagecloud

Support Macs with Apple silicon CPUs

Use new CentOS 7 Docker container for Linux builds

... in order to add SHA-256 payload digests to the official VirtualGL
RPMS, for FIPS compliance.

- Update README.md to reflect that the official Linux binaries now
  require GLIBC 2.17+ across the board.

- Update rpmsign to use the gpg --passphrase option, which eliminates
  the need to use expect.

- Remove the detailed Linux build environment description from
  README.md.  At this point, the Docker recipe is too complex to
  perfectly describe in plain English, and since the recipe is open
  source, such a description is unnecessary.

Use new CentOS 7 Docker container for Linux builds

... in order to add SHA-256 payload digests to the official VirtualGL
RPMS, for FIPS compliance.

- Update README.md to reflect that the official Linux binaries now
  require GLIBC 2.17+ across the board.

- Update rpmsign to use the gpg --passphrase option, which eliminates
  the need to use expect.

- Remove the detailed Linux build environment description from
  README.md.  At this point, the Docker recipe is too complex to
  perfectly describe in plain English, and since the recipe is open
  source, such a description is unnecessary.

Use SHA-256 for YUM repository as well

Use SHA-256 for YUM repository as well

README: The Linux/AArch64 build also needs glibc

Linux: Cross-compile AArch64 packages