[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

git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@373118 91177308-0d34-0410-b5e6-96231b3b80d8

libomptarget/plugins/generic-elf-64bit/src/rtl.cpp

@@ -50,7 +50,7 @@ static int DebugLevel = 0;
 #include "../../common/elf_common.c"
 
 #define NUMBER_OF_DEVICES 4
-#define OFFLOADSECTIONNAME ".omp_offloading.entries"
+#define OFFLOADSECTIONNAME "omp_offloading_entries"
 
 /// Array of Dynamic libraries loaded for this target.
 struct DynLibTy {