Build LLVM with RISCV support

Author: colemancda

utils/build-presets.ini

@@ -1472,7 +1472,6 @@ mixin-preset=
 [preset: LLDB_Nested]
 skip-build-benchmarks
 install-destdir=%(swift_install_destdir)s
-llvm-targets-to-build=X86;ARM;AArch64;PowerPC;SystemZ;Mips
 
 [preset: LLDB_Swift_DebugAssert]
 mixin-preset=

utils/build_swift/build_swift/driver_arguments.py

@@ -1211,7 +1211,7 @@ def create_argument_parser():
     in_group('Build settings specific for LLVM')
 
     option('--llvm-targets-to-build', store,
-           default='X86;ARM;AArch64;PowerPC;SystemZ;Mips',
+           default='X86;ARM;AArch64;PowerPC;SystemZ;Mips;RISCV',
            help='LLVM target generators to build')
 
     option('--llvm-ninja-targets', append,