RFR: 7903697: jextract doesn't use name in asm label attribute [v2]

[Maurizio Cimadamore]

> This PR restores a functionality that was available in a (much) older version of jextract.
> 
> Basically, some library symbols can have a C name that differs from the symbol found in the shared library. This is typically done using an attribute (AsmLabelAttr). We already have code to parse declaration attributes, but when we switched to the new FFM-based jextract, we didn't update code generation to use the aliased name for the symbol lookup, if one was available.
> 
> I've resurrected the old test to check this functionality, with some changes: the original test also tested Windows support, but that seems less important, given that Windows doesn't really have an equivalent feature/attribute.

Maurizio Cimadamore has updated the pull request incrementally with one additional commit since the last revision:

  Tweak header file in test

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

Changes:
  - all: https://git.openjdk.org/jextract/pull/232/files
  - new: https://git.openjdk.org/jextract/pull/232/files/7d07c827..e94bbb85

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jextract&pr=232&range=01
 - incr: https://webrevs.openjdk.org/?repo=jextract&pr=232&range=00-01

  Stats: 6 lines in 1 file changed: 0 ins; 4 del; 2 mod
  Patch: https://git.openjdk.org/jextract/pull/232.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/232/head:pull/232

PR: https://git.openjdk.org/jextract/pull/232