[jdk21u-dev] RFR: 8343452: Incorrect WINDOWS build variable is used in macroAssembler_x86.cpp

[Satyen Subramaniam]

Backporting JDK-8343452: Incorrect WINDOWS build variable is used in macroAssembler_x86.cpp. HotSpot VM build macro only defines _WINDOWS env variable (not WINDOWS). `call_clobbered_gp_registers()` and `call_clobbered_xmm_registers()` use the wrong variable name. Leads to RSI and RDI missing from list of clobbered register on windows due, and may cause issue when calling native code which modifies them. Ran GHA Sanity Checks and local Tier 1 and 2 (though not on a Windows machine). Patch is clean.

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Commit messages:
 - Backport 0e1c1b793d43064aabe9571057284899c9580f30

Changes: https://git.openjdk.org/jdk21u-dev/pull/1848/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=1848&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343452
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/1848.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/1848/head:pull/1848

PR: https://git.openjdk.org/jdk21u-dev/pull/1848