[sanitizer_common] Fix DenseMapCustomTest.DefaultMinReservedSizeTest on SPARC

As described in Issue #53523, the
`DenseMapCustomTest.DefaultMinReservedSizeTest` test FAILs on Solaris/SPARC
(both 32 and 64-bit):

  /vol/llvm/src/llvm-project/local/compiler-rt/lib/sanitizer_common/tests/sanitizer_dense_map_test.cpp:399:
Failure
  Expected: (MemorySize) != (Map.getMemorySize()), actual: 8192 vs 8192

This happens because SPARC, unlike many other CPUs, uses an 8 kB pagesize.

Fixed by incorporating the pagesize into the calculations of
`ExpectedInitialBucketCount` and derived values.

Tested on `sparcv9-sun-solaris2.11`, `amd64-pc-solaris2.11`, and
`x86_64-pc-linux-gnu`.

Differential Revision: https://reviews.llvm.org/D118771