Auto-enable TLSDESC support

TLSDESC speeds up access to dynamic TLS. This is especially important
for non-glibc targets, but is also helpful for non-initial-exec TLS
variables.

The entry asm does not support TLSDESC, but it only accesses
initial-exec symbols, so it is not necessary to handle that separately.

Acked-by: Tapani Pälli <tapani.palli@intel.com>
Acked-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <mesa/mesa!12722>