Build custom allocator/tcmalloc respecting 'use_allocator' flag

Build custom allocator/tcmalloc respecting 'use_allocator' flag

Right now chromium-efl builds TCMalloc unconditonally for
all targets.
However, it is known that some targets, including ARM64,
are not yet supported by TCMalloc, and should be disabled.

Patch prepares the ground for this, only building allocator
the respecting 'use_allocator' GYP define. Now, it will
always build, but soon disabled for ARM64 builds.

Bug: web.sec.samsung.net/bugzilla/show_bug.cgi?id=11564
Reviewed by: Piotr Tworek, SeungSeop Park, arno renevier

Change-Id: I3c087d9782860c3645f4f7763b6e0ae2a4cec195
Signed-off-by: Antonio Gomes <a1.gomes@samsung.com>