std_detect: Always avoid dlsym on *-linux-{musl,ohos}* targets

[taiki-e]

Similar to #1375 (for *-linux-gnu*) and #1406 (for *-android*), but for -linux-{musl,ohos}*:

• *-linux-musl* targets (at least since Rust 1.15) use musl newer than musl 1.1.0 that added getauxval
• *-linux-ohos* targets use a fork of musl 1.2

On musl with static linking, there is a known problem that getauxval is not always available, independent of version requirements: rust-lang/rust#89626
However, this seems to be due to the fact that compiler-builtins is built before libc (which is a dependency of std) links musl. So IIUC the std and its dependent can use getauxval without this problem (std_detect is a part of std). Actually std uses it since Rust 1.78: rust-lang/rust@9da004e

---

After this PR, remaining linux/android targets that use dlsym are:

• 32-bit *-android*: This is actually a issue in libc crate std_detect: Do not use libc::getauxval on 32-bit Android #1421
  • Opened android: Add getauxval for 32-bit targets libc#4338 to solve that issue.
• *-linux-uclibc*: uclibc-ng supports getauxval since 1.0.43
  • This is not available in some years old toolchains mentioned in the examples in the platform support documentation.
    • By the way, since Rust 1.78, the std cannot be compiled with these toolchains without a patch because the commit mentioned above that started using getauxval is included. (Either the std or the platform support documentation should be fixed.)
• *-linux-none: This target does not use any libc APIs.
  • However, at this time, it does not support std in the first place and only for x86_64 which doesn't use getauxval.
  • By the way, mustang provides getauxval.

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[Amanieu]

There's a lot of duplication in this function which makes it hard to review. I think this is a good opportunity to move the choice of whether to use libc::getauxval or look it up dynamically into the local getauxval function.