[libc] implement sigsetjmp/siglongjmp for riscv (#137992)

See https://godbolt.org/z/jo7s6j7sq for compiled code.

```c++
#if __riscv_xlen == 64
#define STORE(A, B, C) "sd " #A ", %c[" #B "](" #C ")\n\t"
#define LOAD(A, B, C) "ld " #A ", %c[" #B "](" #C ")\n\t"
#elif __riscv_xlen == 32
#define STORE(A, B, C) "sw " #A ", %c[" #B "](" #C ")\n\t"
#define LOAD(A, B, C) "lw " #A ", %c[" #B "](" #C ")\n\t"
#else
#error "Unsupported RISC-V architecture"
#endif

namespace LIBC_NAMESPACE_DECL {
[[gnu::naked]]
LLVM_LIBC_FUNCTION(int, sigsetjmp, (sigjmp_buf, int)) {
  // clang-format off
  asm("beqz a1, .Lnosave\n\t"
      STORE(ra, retaddr, a0)
      STORE(s0, extra, a0)
      "mv s0, a0\n\t"
      "call %c[setjmp]\n\t"
      "mv a1, a0\n\t"
      "mv a0, s0\n\t"
      LOAD(s0, extra, a0)
      LOAD(ra, retaddr, a0)
      "tail %c[epilogue]\n"
".Lnosave:\n\t"
      "tail %c[setjmp]"
      // clang-format on
      ::[retaddr] "i"(offsetof(__jmp_buf, sig_retaddr)),
      [extra] "i"(offsetof(__jmp_buf, sig_extra)), [setjmp] "i"(setjmp),
      [epilogue] "i"(sigsetjmp_epilogue)
      : "a0", "a1", "s0");
}
```

libc/config/linux/riscv/entrypoints.txt

@@ -877,6 +877,8 @@ if(LLVM_LIBC_FULL_BUILD)
     # setjmp.h entrypoints
     libc.src.setjmp.longjmp
     libc.src.setjmp.setjmp
+    libc.src.setjmp.siglongjmp
+    libc.src.setjmp.sigsetjmp
 
     # stdio.h entrypoints
     libc.src.stdio.clearerr

libc/include/llvm-libc-types/jmp_buf.h

@@ -12,7 +12,8 @@
 // TODO: implement sigjmp_buf related functions for other architectures
 // Issue: https://github.com/llvm/llvm-project/issues/136358
 #if defined(__linux__)
-#if defined(__i386__) || defined(__x86_64__) || defined(__aarch64__)
+#if defined(__i386__) || defined(__x86_64__) || defined(__aarch64__) ||        \
+    defined(__riscv)
 #define __LIBC_HAS_SIGJMP_BUF
 #endif
 #endif

libc/src/setjmp/riscv/CMakeLists.txt

@@ -11,6 +11,22 @@ add_entrypoint_object(
     -fomit-frame-pointer
 )
 
+if (TARGET libc.src.setjmp.sigsetjmp_epilogue)
+  add_entrypoint_object(
+    sigsetjmp
+    SRCS
+      sigsetjmp.cpp
+    HDRS
+      ../sigsetjmp.h
+    DEPENDS
+      libc.hdr.types.jmp_buf
+      libc.hdr.types.sigset_t
+      libc.hdr.offsetof_macros
+      libc.src.setjmp.sigsetjmp_epilogue
+      libc.src.setjmp.setjmp
+  )
+endif()
+
 add_entrypoint_object(
   longjmp
   SRCS

libc/src/setjmp/riscv/sigsetjmp.cpp

@@ -0,0 +1,49 @@
+//===-- Implementation of sigsetjmp ---------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/setjmp/sigsetjmp.h"
+#include "hdr/offsetof_macros.h"
+#include "src/__support/common.h"
+#include "src/__support/macros/config.h"
+#include "src/setjmp/setjmp_impl.h"
+#include "src/setjmp/sigsetjmp_epilogue.h"
+
+#if __riscv_xlen == 64
+#define STORE(A, B, C) "sd " #A ", %c[" #B "](" #C ")\n\t"
+#define LOAD(A, B, C) "ld " #A ", %c[" #B "](" #C ")\n\t"
+#elif __riscv_xlen == 32
+#define STORE(A, B, C) "sw " #A ", %c[" #B "](" #C ")\n\t"
+#define LOAD(A, B, C) "lw " #A ", %c[" #B "](" #C ")\n\t"
+#else
+#error "Unsupported RISC-V architecture"
+#endif
+
+namespace LIBC_NAMESPACE_DECL {
+[[gnu::naked]]
+LLVM_LIBC_FUNCTION(int, sigsetjmp, (sigjmp_buf, int)) {
+  // clang-format off
+  asm("beqz a1, .Lnosave\n\t"
+      STORE(ra, retaddr, a0)
+      STORE(s0, extra, a0)
+      "mv s0, a0\n\t"
+      "call %c[setjmp]\n\t"
+      "mv a1, a0\n\t"
+      "mv a0, s0\n\t"
+      LOAD(s0, extra, a0)
+      LOAD(ra, retaddr, a0)
+      "tail %c[epilogue]\n"
+".Lnosave:\n\t"
+      "tail %c[setjmp]"
+      // clang-format on
+      ::[retaddr] "i"(offsetof(__jmp_buf, sig_retaddr)),
+      [extra] "i"(offsetof(__jmp_buf, sig_extra)), [setjmp] "i"(setjmp),
+      [epilogue] "i"(sigsetjmp_epilogue)
+      : "a0", "a1", "s0");
+}
+
+} // namespace LIBC_NAMESPACE_DECL