lib/std: support riscv64

Borrow the ucontext layout from Go (probably reasonable?) and fix up
some missing imports for shared structures.

I can compile a glibc-linked hello.zig with:

   zig build-exe hello.zig -lc -target riscv64-linux-gnu.2.38 -mcpu=generic_rv64

or a native Zig binary with:

   zig build-exe hello.zig -target riscv64-linux -mcpu=generic_rv64

But the resulting binary segfaults immediately in my (perhaps broken) qemu
setup.  I've no experience in riscv64, so this maybe be broken in subtle
ways, but perhaps its a useful point for someone with the right hardware
to make more progress.

Builds on ziglang#18803

lib/std/debug.zig

@@ -194,7 +194,7 @@ pub fn dumpCurrentStackTrace(start_addr: ?usize) void {
 
 pub const have_ucontext = @hasDecl(os.system, "ucontext_t") and
     (builtin.os.tag != .linux or switch (builtin.cpu.arch) {
-    .mips, .mipsel, .mips64, .mips64el, .riscv64 => false,
+    .mips, .mipsel, .mips64, .mips64el => false,
     else => true,
 });
 

lib/std/os/linux/riscv64.zig

@@ -9,6 +9,8 @@ const pid_t = std.os.linux.pid_t;
 const sockaddr = linux.sockaddr;
 const socklen_t = linux.socklen_t;
 const timespec = std.os.linux.timespec;
+const stack_t = linux.stack_t;
+const sigset_t = linux.sigset_t;
 
 pub fn syscall0(number: SYS) usize {
     return asm volatile ("ecall"
@@ -223,3 +225,56 @@ pub const Stat = extern struct {
 pub const Elf_Symndx = u32;
 
 pub const VDSO = struct {};
+
+pub const userregs_t = extern struct {
+    pc: u64,
+    ra: u64,
+    sp: u64,
+    gp: u64,
+    tp: u64,
+    t0: u64,
+    t1: u64,
+    t2: u64,
+    s0: u64,
+    s1: u64,
+    a0: u64,
+    a1: u64,
+    a2: u64,
+    a3: u64,
+    a4: u64,
+    a5: u64,
+    a6: u64,
+    a7: u64,
+    s2: u64,
+    s3: u64,
+    s4: u64,
+    s5: u64,
+    s6: u64,
+    s7: u64,
+    s8: u64,
+    s9: u64,
+    s10: u64,
+    s11: u64,
+    t3: u64,
+    t4: u64,
+    t5: u64,
+    t6: u64,
+};
+
+pub const fpregs_t = extern struct {
+    f: [528]u8,
+};
+
+pub const mcontext_t = extern struct {
+    userregs: userregs_t,
+    fpregs: fpregs_t,
+};
+
+pub const ucontext_t = extern struct {
+    flags: u64,
+    link: ?*ucontext_t,
+    stack: stack_t,
+    sigmask: sigset_t,
+    reserved: [8]u8 = undefined,
+    mcontext: mcontext_t,
+};