[compiler-rt][Fuchsia] Change GetMaxUserVirtualAddress to invoke syscall (#153309)

LSan was recently refactored to call GetMaxUserVirtualAddress for
diagnostic purposes. This leads to failures for some of our downstream
tests which only run with lsan. This occurs because
GetMaxUserVirtualAddress depends on setting up shadow via a call to
__sanitizer_shadow_bounds, but shadow bounds aren't set for standalone
lsan because it doesn't use shadow. This updates the function to invoke
the same syscall used by __sanitizer_shadow_bounds calls for getting the
memory limit. Ideally this function would only be called once since we
only need to get the bounds once.

More context in https://fxbug.dev/437346226.

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