[llvm-exegesis] Use mmap2 when mmap is unavailable to fix riscv32 build

Some 32-bit architectures don't have mmap and define mmap2 instead.
E.g. on riscv32 we may get

```
| /mnt/b/yoe/master/build/tmp/work-shared/llvm-project-source-17.0.0-r0/git/llvm/tools/llvm-exegesis/lib/X86/Target.cpp:1116:19: error: use of undeclared identifier 'SYS_mmap'
|  1116 |   generateSyscall(SYS_mmap, MmapCode);
|       |                   ^
| /mnt/b/yoe/master/build/tmp/work-shared/llvm-project-source-17.0.0-r0/git/llvm/tools/llvm-exegesis/lib/X86/Target.cpp:1134:19: error: use of undeclared identifier 'SYS_mmap'
|  1134 |   generateSyscall(SYS_mmap, GeneratedCode);                                                                                                                                       |       |                   ^
| 1 warning and 2 errors generated.
```

Co-Authored-By: Fangrui Song <i@maskray.me>
Differential Revision: https://reviews.llvm.org/D158375