[llvm-exegesis] Close file descriptors after use (#86584)

There are several insstances in the subprocess executor in llvm-exegesis
where we fail to close file descriptors after using them. This leaves
them open, which can cause issues later on if a third-party binary is
using the exegesis libraries and executing many blocks before exiting.
Leaving the descriptors open until process exit is also bad practice.
This patch fixes that by explicitly calling close() when we are done
with a specific file descriptor.

This patch fixes #86583.