Fail to build with musl

I'm cross compiling `abseil(version 20211102.1)` on Linux from x64 to arm with libc `musl(version 1.2.3)` libc.
It failed to build because of function `GetProgramCounter` in `absl/debugging/internal/examine_stack.cc`, which uses `ucontext` to get the `pc` register. In this function, `abseil` use macros to check the platform and use the member of `struct ucontext_t`, so the behavior depends on the implementation of `glibc`'s `ucontext_t`. Meanwhile, `ucontext` **is deprecated** by `POSIX`, so `musl` doesn't implement it.
On current master branch this problem is still exist:
https://github.com/abseil/abseil-cpp/blob/d3f0c70673ed71ba1581702bbbd0aa8865a575d1/absl/debugging/internal/examine_stack.cc#L151-L234