Add CentOS 7.4 to release RPM builds

[bhipple]

This provides RPMs for CentOS 7.4 users, and their counterparts on the
enterprise release RHEL 7.4.

Since CentOS/RHEL 7.4 are based off gcc 4.8.x, they do not have a compliant c++14 compiler available by default for building RPMs. Patchelf does not currently require this and builds fine, but it's worth noting that merging this PR will ensure that that remains the case until the next CentOS/RHEL versions are released.

[bhipple]

$ nix-build release.nix -A rpm_centos74x86_64
these derivations will be built:
  /nix/store/nalcq13glfbl8wf04r1rrdqdh2hngyd9-patchelf-tarball-0.10pre1234_abcdef.drv
  /nix/store/5sx1w5lfyyrhrw4a3pb8x0c1dfiwirhz-patchelf-rpm-centos-7.4-x86_64-0.10pre1234_abcdef.drv
building '/nix/store/nalcq13glfbl8wf04r1rrdqdh2hngyd9-patchelf-tarball-0.10pre1234_abcdef.drv'...
u

...

installing /nix/store/8whir4wbcsccg6d8nsr4rsas4m61mjad-patchelf-rpm-centos-7.4-x86_64-0.10pre1234_abcdef/rpms/centos-7.4-x86_64/patchelf-0.10pre1234_abcdef-1.x86_64.rpm /nix/store/8whir4wbcsccg6d8nsr4rsas4m61mjad-patchelf-rpm-centos-7.4-x86_64-0.10pre1234_abcdef/rpms/centos-7.4-x86_64/patchelf-debuginfo-0.10pre1234_abcdef-1.x86_64.rpm...
uninstalling patchelf-0.10pre1234_abcdef-1.x86_64 patchelf-debuginfo-0.10pre1234_abcdef-1.x86_64...
[    9.710315] reboot: Power down
/nix/store/8whir4wbcsccg6d8nsr4rsas4m61mjad-patchelf-rpm-centos-7.4-x86_64-0.10pre1234_abcdef