openmp: -foffload-memory=pinned

openmp: -foffload-memory=pinned

Implement the -foffload-memory=pinned option such that libgomp is
instructed to enable fully-pinned memory at start-up.  The option is
intended to provide a performance boost to certain offload programs without
modifying the code.

This feature only works on Linux, at present, and simply calls mlockall to
enable always-on memory pinning.  It requires that the ulimit feature is
set high enough to accommodate all the program's memory usage.

In this mode the ompx_pinned_memory_alloc feature is disabled as it is not
needed and may conflict.

Backport of the patch posted at
https://gcc.gnu.org/pipermail/gcc-patches/2022-March/591354.html

gcc/ChangeLog:

	* omp-low.cc (omp_enable_pinned_mode): New function.
	(execute_lower_omp): Call omp_enable_pinned_mode.

libgomp/ChangeLog:

	* config/linux/allocator.c (always_pinned_mode): New variable.
	(GOMP_enable_pinned_mode): New function.
	(linux_memspace_alloc): Disable pinning when always_pinned_mode set.
	(linux_memspace_calloc): Likewise.
	(linux_memspace_free): Likewise.
	(linux_memspace_realloc): Likewise.
	* libgomp.map (GOMP_5.1.1): New version space with
	GOMP_enable_pinned_mode.
	* testsuite/libgomp.c/alloc-pinned-7.c: New test.

gcc/testsuite/ChangeLog:

	* c-c++-common/gomp/alloc-pinned-1.c: New test.