[libc] Implement alloca.h header (#210150)
Implemented the alloca.h header for LLVM libc.
* libc/include/llvm-libc-macros/alloca-macros.h: Defined alloca macro
using __builtin_alloca.
* libc/include/alloca.yaml: Added header specification.
* libc/include/CMakeLists.txt,
libc/include/llvm-libc-macros/CMakeLists.txt: Registered the new header
and macro targets.
* libc/config/linux/*/headers.txt: Enabled the header for Linux targets
(x86_64, aarch64, arm, i386, riscv).
* libc/test/include/alloca_test.cpp, libc/test/include/CMakeLists.txt:
Added and registered functional unit tests.
Tested by running the new unit and hermetic tests.
Assisted-by: Automated tooling, human reviewed.
