[sanitizer_common] Don't try to unmap unaligned memory

[sanitizer_common] Don't try to unmap unaligned memory

Enabling `sanitizer_common` tests on Solaris (D91606
<https://reviews.llvm.org/D91606>) and SPARC (D91608
<https://reviews.llvm.org/D91608>) uncovered a sparcv9 failure

  SanitizerCommon-Unit :: ./Sanitizer-sparcv9-Test/CompactRingBuffer.int64

like this:

  [ RUN      ] CompactRingBuffer.int64
  ==24576==ERROR: SanitizerTool failed to deallocate 0x2000 (8192) bytes at address 0xffffffff7f59b000
  ==24576==Sanitizer CHECK failed: /vol/llvm/src/llvm-project/local/compiler-rt/lib/sanitizer_common/sanitizer_posix.cpp:61 (("unable to unmap" && 0)) != (0) (0, 0)

The problem is that the original allocation via
`MmapAlignedOrDieOnFatalError` is for 4 kB, but the Solaris/sparcv9
pagesize is 8 kB.  So the initial allocation is for 12 kB, rounded to a
multiple of the pagesize.  Afterwards, the unneeded rest is unmapped again,
but this fails since the address is not pagesize-aligned.

This patch avoids this by aligning the end of the mapping to the pagesize.

With D91827 <https://reviews.llvm.org/D91827> added, the test `PASS`es on
`sparcv9-sun-solaris2.11`.

Differential Revision: https://reviews.llvm.org/D91615