[libc] Enable spawn lib in riscv

In this patch we add support for the spawn lib in riscv.

Only small changes were required, the biggest one was to use of dup3
instead of dup2, if the latter is not available. This follows our
implementation of dup2.

Differential Revision: https://reviews.llvm.org/D146145

libc/config/linux/riscv64/entrypoints.txt

@@ -431,6 +431,14 @@ if(LLVM_LIBC_FULL_BUILD)
     libc.src.signal.sigfillset
     libc.src.signal.signal
 
+    # spawn.h entrypoints
+    libc.src.spawn.posix_spawn
+    libc.src.spawn.posix_spawn_file_actions_addclose
+    libc.src.spawn.posix_spawn_file_actions_adddup2
+    libc.src.spawn.posix_spawn_file_actions_addopen
+    libc.src.spawn.posix_spawn_file_actions_destroy
+    libc.src.spawn.posix_spawn_file_actions_init
+
     # threads.h entrypoints
     libc.src.threads.call_once
     libc.src.threads.cnd_broadcast

libc/config/linux/riscv64/headers.txt

@@ -9,6 +9,7 @@ set(TARGET_PUBLIC_HEADERS
     libc.include.pthread
     libc.include.sched
     libc.include.signal
+    libc.include.spawn
     libc.include.stdio
     libc.include.stdlib
     libc.include.string

libc/src/spawn/linux/posix_spawn.cpp

@@ -14,6 +14,7 @@
 #include "src/spawn/file_actions.h"
 
 #include <fcntl.h>
+#include <signal.h> // For SIGCHLD
 #include <spawn.h>
 #include <sys/syscall.h> // For syscall numbers.
 
@@ -50,8 +51,15 @@ cpp::optional<int> open(const char *path, int oflags, mode_t mode) {
 
 void close(int fd) { __llvm_libc::syscall_impl(SYS_close, fd); }
 
+// We use dup3 if dup2 is not available, similar to our implementation of dup2
 bool dup2(int fd, int newfd) {
+#ifdef SYS_dup2
   long ret = __llvm_libc::syscall_impl(SYS_dup2, fd, newfd);
+#elif defined(SYS_dup3)
+  long ret = __llvm_libc::syscall_impl(SYS_dup3, fd, newfd, 0);
+#else
+#error "SYS_dup2 and SYS_dup3 not available for the target."
+#endif
   return ret < 0 ? false : true;
 }