[llvm] [Support] Vendor rpmalloc in-tree and use it for the Windows 64-bit release (PR #91862)

[Alexandre Ganea via llvm-commits]

aganea wrote:

Just for reference, latest MSVC 17.9.7 is litteraly twice as slow to build LLVM, as compared to this current PR:
```
C:\src\git\llvm-project>hyperfine -r 5 -p "make_llvm.bat stage1_test2" "ninja clang -C stage1_test2"
Benchmark 1: ninja clang -C stage1_test2
  Time (mean ± σ):     784.241 s ± 17.021 s    [User: 28043.391 s, System: 2018.822 s]
  Range (min … max):   761.882 s … 804.272 s    5 runs
```
Some of the LLVM source files seem to be hitting a quadratic behavior somewhere in MSVC: https://developercommunity.visualstudio.com/t/ON2-in-SparseBitVectorBase-when-com/10657991

https://github.com/llvm/llvm-project/pull/91862