[libc] Implement getcontext and setcontext for x86_64 (#192343)

Implemented getcontext and setcontext for x86_64 architecture in LLVM
libc. These functions use inline assembly with naked attributes to
capture and restore the exact register state.

Added:
* src/ucontext/getcontext.h and setcontext.h
* src/ucontext/x86_64/getcontext.cpp and setcontext.cpp
* Hermetic integration test for register preservation.
* Unit tests for basic functionality and signal mask preservation.

Updated entrypoints for x86_64 Linux.

GitOrigin-RevId: b2e0403b11de1ce60fbf2aadec0cc4efb44cd4c4