RISC-V: Disable use of TLS copy relocs.

RISC-V: Disable use of TLS copy relocs.

Musl and lld don't support TLS copy relocs, and don't want to add support
for this feature which is unique to RISC-V.  Only GNU ld and glibc support
them.  In the pasbi discussion, people have pointed out various problems
with using them, so we are deprecating them.  There doesn't seem to be an
ABI break from dropping them so this patch modifies gcc to stop creating
them.  I'm using an ifdef for now in case a problem turns up and the code
has to be re-enabled.  The plan is to add an initial to local exec
relaxation as a replacement, though this has not been defined or
implemented yet.

	gcc/
	* config/riscv/riscv.c (riscv_legitimize_tls_address): Ifdef out
	use of TLS_MODEL_LOCAL_EXEC when not pic.

From-SVN: r280025