[llvm] [llvm-exegesis] Add support for pinning benchmarking process to a CPU (PR #85168)

[Mikael Holmén via llvm-commits]

mikaelholmen wrote:

This patch broke trunk for me.
I compile on a RHEL8 machine with clang 15.0.5 and I get
```
../tools/llvm-exegesis/lib/BenchmarkRunner.cpp:416:12: error: use of undeclared identifier 'getcpu'; did you mean 'getcwd'?
    assert(getcpu(&CurrentCPU, nullptr) == 0 &&
           ^~~~~~
           getcwd
/usr/include/assert.h:90:27: note: expanded from macro 'assert'
     (static_cast <bool> (expr)                                         \
                          ^
/usr/include/unistd.h:511:14: note: 'getcwd' declared here
extern char *getcwd (char *__buf, size_t __size) __THROW __wur;
             ^
../tools/llvm-exegesis/lib/BenchmarkRunner.cpp:416:19: error: cannot initialize a parameter of type 'char *' with an rvalue of type 'unsigned int *'
    assert(getcpu(&CurrentCPU, nullptr) == 0 &&
                  ^~~~~~~~~~~
/usr/include/assert.h:90:27: note: expanded from macro 'assert'
     (static_cast <bool> (expr)                                         \
                          ^~~~
/usr/include/unistd.h:511:28: note: passing argument to parameter '__buf' here
extern char *getcwd (char *__buf, size_t __size) __THROW __wur;
                           ^
2 errors generated.
```
Similar to the buildbot failure in the previous comment.

https://github.com/llvm/llvm-project/pull/85168