meson: Additionally probe -mtls-dialect=desc for TLSDESC support
Previously only `-mtls-dialect=gnu2` was probed, which was appropriate for arm, x86 and x86_64, but not for newer architectures such as aarch64, loongarch64 and riscv64 which all use `-mtls-dialect=desc` instead. Because the driver option is not consistent across architectures (and probably will not), try both variants and choose the first one working. While at it, rename "gnu2_*" variables to "tlsdesc_*" respectively, for clarity. Cc: mesa-stable Reviewed-by:Icenowy Zheng <uwu@icenowy.me> Reviewed-by:
Yukari Chiba <i@0x7f.cc> Reviewed-by:
David Heidelberg <david@ixit.cz> Signed-off-by:
WANG Xuerui <git@xen0n.name> Part-of: <mesa/mesa!30599> (cherry picked from commit cc2dbb8e)