[sanitizer_common] Change allocator base in test case for compatibili… (#93234)

…ty with high-entropy ASLR

With high-entropy ASLR (e.g., 32-bits == 16TB), the allocator base of
0x700000000000 (112TB) may collide with the placement of the libraries
(e.g., on Linux, the mmap base could be 128TB - 16TB == 112TB). This
results in a segfault in the test case.

This patch moves the allocator base below the PIE program segment,
inspired by fb77ca05. As per that patch:
1) we are leaving the old behavior for Apple 2) since ASLR cannot be set
above 32-bits for x86-64 Linux, we expect this new layout to be durable.

Note that this is only changing a test case, not the behavior of
sanitizers. Sanitizers have their own settings for initializing the
allocator base.

Reproducer:
1. ninja check-sanitizer # Just to build the test binary needed below;
no need to actually run the tests here
2. sudo sysctl vm.mmap_rnd_bits=32 # Increase ASLR entropy
3. for f in `seq 1 10000`; do echo $f;
GTEST_FILTER=*SizeClassAllocator64Dense
./projects/compiler-rt/lib/sanitizer_common/tests/Sanitizer-x86_64-Test
> /tmp/x; if [ $? -ne 0 ]; then cat /tmp/x; fi; done