x64: Allow building with LUAJIT_USE_SYSMALLOC and LUAJIT_USE_VALGRIND.

Valgrind 3.9 killed MAP_32BIT support. Ugh. So now we have to rely on
undocumented behavior where Valgrind always allocates from the bottom
of memory. Alas, such a binary won't run properly without Valgrind.
Prevent adding side traces for stack checks.
Prevent GC estimate miscalculation due to buffer growth.
Merge branch 'master' into v2.1