libexecinfo: Handle machine-dependent additions to expected symbols.

Needed for hppa.

PR lib/58838: shared libraries in base should all have expsym lists

Decode function descriptors (aka plabels) on hppa to fix backtrace.

PR/56881: hppa: backtrace_symbols() delivers bogus results for some dynloaded functions

Install gcc's unwind.h instead of libexecinfo's

The prototypes in libexecinfo's unwind.h do not match those commonly
used (e.g. by gcc, clang, GNU libunwind, LLVM libunwind...), causing
C++ programs

Install gcc's unwind.h instead of libexecinfo's

The prototypes in libexecinfo's unwind.h do not match those commonly
used (e.g. by gcc, clang, GNU libunwind, LLVM libunwind...), causing
C++ programs to fail to build on type mismatches (e.g. compiler-rt,
libc++abi). Rather than providing our own header, reuse the one
included in gcc.

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Don't overwrite the real unwind.h if it is present.

don't kill the existing includes

Make unwind.h installable.

another libelf instance

earm -> earm*

Changes for MKCOMPAT for earm.

When building for earm, make sure to build unwind_arm_ehabi_stub.*

Add a similar to linux backtrace_* and execinfo.h family of functions