RFR: 8367485: os::physical_memory is broken in 32-bit JVMs when running on 64-bit OSes

[Anton Artemov]

Hi, please consider the following changes:

In this PR we address the overflow issue in `os::physical_memory()` on Linux, which can occur when running a 32-bit JVM on a 64-bit machine, introduced by https://bugs.openjdk.org/browse/JDK-8357086. The problem is that the product of _SC_PHYS_PAGES and _SC_PAGESIZE can overflow according to the documentation. 

The issue is addressed by checking if the product (represented as `uint64_t`) exceeds the maximum value representable by `size_t`, and, if so `std::numeric_limits<size_t>::max()` is returned as physical memory. If there is no overflow detected, the product of _SC_PHYS_PAGES  and _SC_PAGESIZE  is returned. 

Note that this change does not roll back to the old behavior (pre https://bugs.openjdk.org/browse/JDK-8357086). We keep consistency among memory functions return types.

Tested in tiers 1 - 3.

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Commit messages:
 - 8367485: Whitespace fix.
 - 8367485: Fixed overflow issue in os::physical_memory on Linux.

Changes: https://git.openjdk.org/jdk/pull/27335/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27335&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8367485
  Stats: 8 lines in 1 file changed: 7 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/27335.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27335/head:pull/27335

PR: https://git.openjdk.org/jdk/pull/27335