POSIX compliant poll.h and mmap over sys/poll.h and mmap64

[SamuelMarks]

Closes #10737

Basic build testing confirms this works with both musl and glibc:

FROM alpine
RUN apk add --no-cache cmake gcc g++ make musl-dev
WORKDIR /src/xgboost
COPY . .
RUN cmake -S . -B build && \
    cmake --build build

FROM debian:bookworm-slim
RUN apt-get update -qq && \
    apt-get install -y cmake gcc g++ make libstdc++6 && \
    rm -rf /var/lib/apt/lists/*
WORKDIR /src/xgboost
COPY . .
RUN cmake -S . -B build && \
    cmake --build build

[trivialfis]

Thank you for the fix!

[snoopcheri]

Great to see this fixed, as we stumbled over this problem ourselves as well!
I assume this will be part of the 2.2.x release?

[trivialfis]

We will probably make a patch release.