Original PR description:
```
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.
```
Note that we are currently requiring glibc >= 2.19.