malloc: Add Huge Page support for mmap

With the morecore hook removed, there is not easy way to provide huge
pages support on with glibc allocator without resorting to transparent
huge pages.  And some users and programs do prefer to use the huge pages
directly instead of THP for multiple reasons: no splitting, re-merging
by the VM, no TLB shootdowns for running processes, fast allocation
from the reserve pool, no competition with the rest of the processes
unlike THP, no swapping all, etc.

This patch extends the 'glibc.malloc.hugetlb' tunable: the value
'2' means to use huge pages directly with the system default size,
while a positive value means and specific page size that is matched
against the supported ones by the system.

Currently only memory allocated on sysmalloc() is handled, the arenas
still uses the default system page size.

To test is a new rule is added tests-malloc-hugetlb2, which run the
addes tests with the required GLIBC_TUNABLE setting.  On systems without
a reserved huge pages pool, is just stress the mmap(MAP_HUGETLB)
allocation failure.  To improve test coverage it is required to create
a pool with some allocated pages.

Checked on x86_64-linux-gnu.

Reviewed-by: DJ Delorie <dj@redhat.com>

sysdeps/generic/malloc-hugepages.h

@@ -34,4 +34,11 @@ enum malloc_thp_mode_t
 
 enum malloc_thp_mode_t __malloc_thp_mode (void) attribute_hidden;
 
+/* Return the supported huge page size from the REQUESTED sizes on PAGESIZE
+   along with the required extra mmap flags on FLAGS,  Requesting the value
+   of 0 returns the default huge page size, otherwise the value will be
+   matched against the sizes supported by the system.  */
+void __malloc_hugepage_config (size_t requested, size_t *pagesize, int *flags)
+     attribute_hidden;
+
 #endif /* _MALLOC_HUGEPAGES_H */