[clang] [clang][RISCV] Add big-endian RISC-V target support (PR #165599)

[via cfe-commits]

gulfemsavrun wrote:

We started seeing the test failure (`Clang :: Driver/riscv-be.c`) after this commit:
```

FAIL: Clang :: Driver/riscv-be.c (13392 of 23710)
******************** TEST 'Clang :: Driver/riscv-be.c' FAILED ********************
Exit Code: 1

Command Output (stdout):
--
# RUN: at line 3
/b/s/w/ir/x/w/llvm_build/bin/clang -target riscv64be-unknown-elf -### /b/s/w/ir/x/w/llvm-llvm-project/clang/test/Driver/riscv-be.c 2>&1    | /b/s/w/ir/x/w/llvm_build/bin/FileCheck /b/s/w/ir/x/w/llvm-llvm-project/clang/test/Driver/riscv-be.c
# executed command: /b/s/w/ir/x/w/llvm_build/bin/clang -target riscv64be-unknown-elf '-###' /b/s/w/ir/x/w/llvm-llvm-project/clang/test/Driver/riscv-be.c
# executed command: /b/s/w/ir/x/w/llvm_build/bin/FileCheck /b/s/w/ir/x/w/llvm-llvm-project/clang/test/Driver/riscv-be.c
# RUN: at line 5
/b/s/w/ir/x/w/llvm_build/bin/clang -target riscv64be-unknown-elf -Wno-riscv-be-experimental -### /b/s/w/ir/x/w/llvm-llvm-project/clang/test/Driver/riscv-be.c 2>&1    | /b/s/w/ir/x/w/llvm_build/bin/FileCheck /b/s/w/ir/x/w/llvm-llvm-project/clang/test/Driver/riscv-be.c --check-prefix=NOWARN
# executed command: /b/s/w/ir/x/w/llvm_build/bin/clang -target riscv64be-unknown-elf -Wno-riscv-be-experimental '-###' /b/s/w/ir/x/w/llvm-llvm-project/clang/test/Driver/riscv-be.c
# executed command: /b/s/w/ir/x/w/llvm_build/bin/FileCheck /b/s/w/ir/x/w/llvm-llvm-project/clang/test/Driver/riscv-be.c --check-prefix=NOWARN
# RUN: at line 14
/b/s/w/ir/x/w/llvm_build/bin/clang -### /b/s/w/ir/x/w/llvm-llvm-project/clang/test/Driver/riscv-be.c --target=riscv64be-unknown-linux-gnu    -Wno-riscv-be-experimental --rtlib=platform -mabi=lp64d 2>&1    | /b/s/w/ir/x/w/llvm_build/bin/FileCheck -check-prefix=RV64BE-LINUX-LP64D /b/s/w/ir/x/w/llvm-llvm-project/clang/test/Driver/riscv-be.c
# executed command: /b/s/w/ir/x/w/llvm_build/bin/clang '-###' /b/s/w/ir/x/w/llvm-llvm-project/clang/test/Driver/riscv-be.c --target=riscv64be-unknown-linux-gnu -Wno-riscv-be-experimental --rtlib=platform -mabi=lp64d
# note: command had no output on stdout or stderr
# error: command failed with exit status: 1
# executed command: /b/s/w/ir/x/w/llvm_build/bin/FileCheck -check-prefix=RV64BE-LINUX-LP64D /b/s/w/ir/x/w/llvm-llvm-project/clang/test/Driver/riscv-be.c

--

********************

```
https://luci-milo.appspot.com/ui/p/fuchsia/builders/toolchain.ci/clang-linux-x64/b8692666941831732273/overview

https://github.com/llvm/llvm-project/pull/165599