[RISCV] Avoid save-restore target feature warning

Summary:
LLVM issues a warning if passed unknown target features. Neither I nor
@asb noticed this until after https://reviews.llvm.org/D63498 landed.

This patch stops passing the (unknown) "save-restore" target feature to
the LLVM backend, but continues to emit a warning if a driver asks for
`-msave-restore`. The default of assuming `-mno-save-restore` (and
emitting no warnings) remains.

Reviewers: asb

Reviewed By: asb

Subscribers: rbar, johnrusso, simoncook, apazos, sabuasal, niosHD, kito-cheng, shiva0217, jrtc27, zzheng, edward-jones, rogfer01, MartinMosbeck, brucehoult, the_o, rkruppe, PkmX, jocewei, psnobl, benna, Jim, cfe-commits, asb

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64008

llvm-svn: 364777