Now that we require glibc 2.17+, #17538, we can remove linking in librt
for backwards compatibility purposes. The clock_* functions from librt
were merged into glibc as part of the 2.17 release.

* The `clock_*' suite of functions (declared in <time.h>) is now
available
  directly in the main C library.  Previously it was necessary to link
with
  -lrt to use these functions.  This change has the effect that a
  single-threaded program that uses a function such as `clock_gettime'
(and
  is not linked with -lrt) will no longer implicitly load the pthreads
  library at runtime and so will not suffer the overheads associated
with
  multi-thread support in other code such as the C++ runtime library.

https://sourceware.org/ml/libc-announce/2012/msg00001.html

Note that librt is already not linked by the RISC-V and AARCH64
binaries.