[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

libunwind/src/UnwindLevel1-gcc-ext.c

@@ -167,7 +167,7 @@ _Unwind_Backtrace(_Unwind_Trace_Fn callback, void *ref) {
     }
 
     // Update the pr_cache in the mock exception object.
-    const uint32_t* unwindInfo = (uint32_t *) frameInfo.unwind_info;
+    uint32_t *unwindInfo = (uint32_t *)frameInfo.unwind_info;
     ex.pr_cache.fnstart = frameInfo.start_ip;
     ex.pr_cache.ehtp = (_Unwind_EHT_Header *) unwindInfo;
     ex.pr_cache.additional= frameInfo.flags;