Add information to .eh_frame ELF secion about make_fcontext function.

[vchigrin]

Clang version of libunwind uses long linear scan of the .eh_frame section if it fails to find frame information in .eh_frame_hdr.
That causes long times of stack unwinding in case when current stack has frames with missed FDE records, so it is better to have information about all frames in the stack up until frame which marks return address ad "undefined/unrecoverable".

[vchigrin]

I am working on PR to clang to make stack walking times better in case of missed frames (See llvm/llvm-project#167849 ). But while generic solution is not ready, small local fix here seems resasonable.

[vchigrin]

On our test binary these changes speed up stack unwinding rather strong. Time of construction of boost::stacktrace::stacktrace object in coroutine was:

Before changes: 5071µs
After changes: 82µs

[olk]

ty