[RISCV] Set triple based on -march flag

For RISC-V the value provided to -march should determine whether to
compile for 32- or 64-bit RISC-V irrespective of the target provided to
the Clang driver. This adds a test for this flag for RISC-V and sets the
Target architecture correctly in these cases.

Differential Revision: https://reviews.llvm.org/D54214
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index aa0d88d..5c8f3bd 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -112,7 +112,8 @@
 Modified Compiler Flags
 -----------------------
 
-- ...
+- RISC-V now sets the architecture (riscv32/riscv64) based on the value provided
+  to the ``-march`` flag, overriding the target provided by ``-triple``.
 
 New Pragmas in Clang
 --------------------