RFR: 7904115: Fix for AIX test case failures due to incorrect alignment for double and pointer

[Varada M]

Total of 10 test failures observed on AIX:
jtreg/generator/nestedTypes/TestNestedTypesUnsupported.java 
jtreg/generator/test8246400/LibTest8246400Test.java 
jtreg/generator/test8258605/LibTest8258605Test.java 
jtreg/generator/test8261511/Test8261511.java 
jtreg/generator/testStruct/LibStructTest.java 
testng/org/openjdk/jextract/test/toolprovider/ConstantsTest.java 
testng/org/openjdk/jextract/test/toolprovider/IncompleteArrayTest.java 
testng/org/openjdk/jextract/test/toolprovider/Test8240811.java 
testng/org/openjdk/jextract/test/toolprovider/TestClassGeneration.java 
testng/org/openjdk/jextract/test/toolprovider/nestedAnonOffset/TestNestedAnonOffset.java

This PR fixes AIX specific layout generation issues related to incorrect alignment double and pointer types.
1. Structs containing double fields fail with:
    i. Unsupported layout: 4%D8
    ii. Invalid alignment constraint for member layout
    double in AIX structs has size 8 but alignment 4 (except as first field). AIX specific handling for C_DOUBLE computes the     correct alignment.

2. Clang was detected as 32-bit due to missing -m64 during macro extraction, causing inconsistent macros. This caused jextract to interpret pointer constants incorrectly, leading to failures like:
expected [-1] but found [4294967295]

3. TestNestedAnonOffset.java test failed on AIX because it also expects more padding similar to platforms like windows and linux


After the patch jtreg tests passes successfully.

JBS: [CODETOOLS-7904115](https://bugs.openjdk.org/browse/CODETOOLS-7904115)

-------------

Commit messages:
 - testcase fixes for aix
 - testcase fixes for aix
 - testcase fixes for aix
 - testcase fixes for aix

Changes: https://git.openjdk.org/jextract/pull/296/files
  Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=296&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7904115
  Stats: 27 lines in 6 files changed: 24 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jextract/pull/296.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/296/head:pull/296

PR: https://git.openjdk.org/jextract/pull/296