[Clang][OpenMP Offload] Create start/end symbols for the offloading entry table with a help of a linker
Linker automatically provides start_<section name> and stop_<section name> symbols to satisfy unresolved references if <section name> is representable as a C identifier (see https://sourceware.org/binutils/docs/ld/Input-Section-Example.html for details). These symbols indicate the start address and end address of the output section respectively. Therefore, renaming OpenMP offload entries section name from ".omp.offloading_entries" to "omp_offloading_entries" to use this feature.
This is the first part of the patch for eliminating OpenMP linker script (please see https://reviews.llvm.org/D64943).
Differential Revision: https://reviews.llvm.org/D68070