[Linux/x86] Use ebp from current context during unwinding (dotnet/coreclr#26789)

[Linux/x86] Use ebp from current context during unwinding (dotnet/coreclr#26789)

pCurrentContextPointers in REGDISPLAY can contain NULLs so we need to use
ebp value from pCurrentContext. This patch contains following changes:

- GetRegdisplayFP returns ebp from pCurrentContext
- GetRegdisplayFP is used instead of *GetEbpLocation()
- Set##reg##Location also updates register value in pCurrentContext

Commit migrated from https://github.com/dotnet/coreclr/commit/1e27a905a0081fe95dd7aaf5f9e99449b4811c1a