[RISCV] Match GCC `-march`/`-mabi` driver defaults

Summary:
Clang/LLVM is a cross-compiler, and so we don't have to make a choice
about `-march`/`-mabi` at build-time, but we may have to compute a
default `-march`/`-mabi` when compiling a program. Until now, each
place that has needed a default `-march` has calculated one itself.

This patch adds a single place where a default `-march` is calculated,
in order to avoid calculating different defaults in different places.

This patch adds a new function `riscv::getRISCVArch` which encapsulates
this logic based on GCC's for computing a default `-march` value
when none is provided. This patch also updates the logic in
`riscv::getRISCVABI` to match the logic in GCC's build system for
computing a default `-mabi`.

This patch also updates anywhere that `-march` is used to now use the
new function which can compute a default. In particular, we now
explicitly pass a `-march` value down to the gnu assembler.

GCC has convoluted logic in its build system to choose a default
`-march`/`-mabi` based on build options, which would be good to match.
This patch is based on the logic in GCC 9.2.0. This commit's logic is
different to GCC's only for baremetal targets, where we default
to rv32imac/ilp32 or rv64imac/lp64 depending on the target triple.

Tests have been updated to match the new logic.

Reviewers: asb, luismarques, rogfer01, kito-cheng, khchen

Reviewed By: asb, luismarques

Subscribers: sameer.abuasal, rbar, johnrusso, simoncook, apazos, sabuasal, niosHD, shiva0217, jrtc27, MaskRay, zzheng, edward-jones, MartinMosbeck, brucehoult, the_o, rkruppe, PkmX, jocewei, psnobl, benna, Jim, s.egerton, pzheng, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D69383

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