MemToShadowImpl() maps lower addresses to a memory space out of sanitizers range. The simplest example is address 0 which is mapped to 0x2000000000 but since
static const uptr kShadowBeg = 0x2400000000ull;
accessing the address during tsan execution will lead to a segmentation fault.
This patch expands the range used by the sanitizer and ensures that 1/8 of the maximum valid address in the virtual address spaces is used for shadow memory.