[libunwind] Fix build error on 32 bit Arm after -Wcast-qual was added

New warning was added in https://reviews.llvm.org/D153911 which caused:
https://buildkite.com/llvm-project/libcxx-ci/builds/28407#01896b79-2a5e-4554-ac31-2abec5a8b281

../../libunwind/src/UnwindLevel1-gcc-ext.c:172:47: error: cast from 'const unsigned int *' to 'unsigned int *' drops const qualifier [-Werror,-Wcast-qual]
    ex.pr_cache.ehtp = (_Unwind_EHT_Header *) unwindInfo;

I don't see any reason there should be a const here in the first place,
so just remove it.

Reviewed By: #libunwind, michaelplatings, MaskRay

Differential Revision: https://reviews.llvm.org/D155685