Initialize tunable list with the GLIBC_TUNABLES environment variable

Initialize tunable list with the GLIBC_TUNABLES environment variable

Read tunables values from the users using the GLIBC_TUNABLES
environment variable.  The value of this variable is a colon-separated
list of name=value pairs.  So a typical string would look like this:

GLIBC_TUNABLES=glibc.malloc.mmap_threshold=2048:glibc.malloc.trim_threshold=1024

	* config.make.in (have-loop-to-function): Define.
	* elf/Makefile (CFLAGS-dl-tunables.c): Add
	-fno-tree-loop-distribute-patterns.
	* elf/dl-tunables.c: Include libc-internals.h.
	(GLIBC_TUNABLES): New macro.
	(tunables_strdup): New function.
	(parse_tunables): New function.
	(min_strlen): New function.
	(__tunables_init): Use the new functions and macro.
	(disable_tunable): Disable tunable from GLIBC_TUNABLES.
	* malloc/tst-malloc-usable-tunables.c: New test case.
	* malloc/tst-malloc-usable-static-tunables.c: New test case.
	* malloc/Makefile (tests, tests-static): Add tests.