Add initial riscv64 port

[am11]

These are the initial configuration for riscv64 support. With this delta, the build status of relevant subsets is as follow:

• mono: build succeeds (debug and release). Debug Logs | Release Logs
• libs.native: build succeeds
• host.native: build succeeds (currently only with -p:RuntimeFlavor=mono, since singlefilehost is stuck due to pending clr support)
• clr: we get to the -- Configuring done point (introspection test, locating dependencies etc. are working). Now we just need to write code starting with PAL. :)

This is done with llvm 15 RC1 on the host, and llvm 14 in rootfs. Currently I am using this tag: mcr.microsoft.com/dotnet-buildtools/prereqs:ubuntu-20.04-cross-riscv64-20220803130642-5291f63 which has clang-9 in base (host) layer, so I installed llvm-15 manually and upstremed the change: dotnet/dotnet-buildtools-prereqs-docker#651.

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Issue Details

---

These are the initial configuration for riscv64 support. With this delta, the build status of relevant subsets is as follow:

• mono: build succeeds (debug and release). Debug Logs | Release Logs
• libs.native: build succeeds
• host.native: build succeeds (currently only with -p:RuntimeFlavor=mono, since singlefilehost is stuck due to pending clr support)
• clr: we get to the -- Configuring done point (introspection test, locating dependencies etc. are working). Now we just need to write code starting with PAL. :)

This is done with llvm 15 RC1 on the host, and llvm 14 in rootfs. Currently I am using this tag: mcr.microsoft.com/dotnet-buildtools/prereqs:ubuntu-20.04-cross-riscv64-20220803130642-5291f63 which has clang-9 in base (host) layer, so I installed llvm-15 manually and upstremed the change: dotnet/dotnet-buildtools-prereqs-docker#651.

Author:	am11
Assignees:	-
Labels:	area-Infrastructure, community-contribution
Milestone:	-

[vargaz]

The mono changes look ok otherwise.

[am11]

These were redundant; set in configurecompiler.cmake

[jkotas]

LGTM. Thanks!