Add CHPL_LLVM_GCC_INSTALL_DIR as an alternative to CHPL_LLVM_GCC_PREFIX

[mppf]

This PR adds CHPL_LLVM_GCC_INSTALL_DIR as an alternative to CHPL_LLVM_GCC_PREFIX.

This PR is motivated by the discussion in https://chapel.discourse.group/t/cannot-make-gpu-enabled-chapel/37046/12 .

CHPL_LLVM_GCC_PREFIX allows one to select a GCC installation. The trouble is, on some systems (E.g. Ubuntu 24.04), there can be multiple versions of GCC installed at the same time, with the same prefix (/usr in this case).

clang actually supports --gcc-install-dir as of clang 16. That flag can be used to request a particular version of GCC on Ubuntu.

To understand what one might want to include:

• run clang++ -v hello.cc where hello.cc might just contain int main() { return 0; }
• you can see the GCC installations it considers at the beginning, e.g.

  Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/11
  Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/12
  Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/13
  Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/14
  Selected GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/14
  

• you can try to change which version should be used, e.g. with clang++ -v hello.cc --gcc-install-dir=/usr/bin/../lib/gcc/x86_64-linux-gnu/13

Reviewed by @e-kayrakli - thanks!

• able to build a GPU-enabled runtime on Ubuntu 24.04 with GCC 14 installed
• full comm=none testing

[e-kayrakli]

Should we ignore the environment variable for clangs that don't support the flag that we use? (clang <16, I think?)

[e-kayrakli]

Looks great, thanks!