add RISC-V support to Mesa easyblock

[bedroge]

Simple change that allows Mesa to be built on RISC-V.

[bedroge]

Test report by @bedroge

Overview of tested easyconfigs (in order)

• SUCCESS gettext-0.22-GCCcore-13.2.0.eb
• SUCCESS Mesa-23.1.9-GCCcore-13.2.0.eb

Build succeeded for 2 out of 2 (1 easyconfigs in total)
starfive - Linux Debian GNU/Linux 2.15, RISC-V-64, UNKNOWN, Python 3.10.9
See https://gist.github.com/bedroge/ab4fffe37fa5c983d409bfe62beec569 for a full test report.

[ocaisa]

LGTM