[Chapel Merge] Document CHPL_LLVM_GCC_PREFIX

Branch: refs/heads/main
Revision: d2525bf
Author: mppf
Link: Unavailable
Log Message:

Merge pull request #19519 from mppf/document-CHPL_LLVM_GCC_PREFIX

Document CHPL_LLVM_GCC_PREFIX

The environment variable CHPL_LLVM_GCC_PREFIX has so far been
undocumented. However, recent experience indicates that sometimes setting
it is necessary to get things working.

So, this PR documents it within the description of the CHPL_LLVM setting.

Note that CHPL_LLVM_GCC_PREFIX is not currently shown with
printchplenv --all and arguably it should be. However this PR focuses
only on documentation.

Reviewed by @daviditen - thanks!

Modified Files:
M doc/rst/usingchapel/chplenv.rst

Compare: https://github.com/chapel-lang/chapel/compare/aa549d76701c...d2525bf7bb8d