fix RISC-V assembly CPU features

Previously, Zig did not properly communicate the target CPU features for
RISC-V to clang assembler, because Clang has a different way to pass CPU
features for C code and for assembly code. This commit makes Zig pass a
RISC-V -march flag in order to communicate CPU features to Clang when
compiling assembly files.

src/Compilation.zig

@@ -3004,11 +3004,50 @@ pub fn addCCArgs(
             // all CPU features here.
             switch (target.cpu.arch) {
                 .riscv32, .riscv64 => {
+                    const RvArchFeat = struct { char: u8, feat: std.Target.riscv.Feature };
+                    const letters = [_]RvArchFeat{
+                        .{ .char = 'm', .feat = .m },
+                        .{ .char = 'a', .feat = .a },
+                        .{ .char = 'f', .feat = .f },
+                        .{ .char = 'd', .feat = .d },
+                        .{ .char = 'c', .feat = .c },
+                    };
+                    const prefix: []const u8 = if (target.cpu.arch == .riscv64) "rv64" else "rv32";
+                    const prefix_len = 4;
+                    assert(prefix.len == prefix_len);
+                    var march_buf: [prefix_len + letters.len]u8 = undefined;
+                    var march_index: usize = prefix_len;
+                    mem.copy(u8, &march_buf, prefix);
+
+                    if (std.Target.riscv.featureSetHas(target.cpu.features, .e)) {
+                        march_buf[march_index] = 'e';
+                    } else {
+                        march_buf[march_index] = 'i';
+                    }
+                    march_index += 1;
+
+                    for (letters) |letter| {
+                        if (std.Target.riscv.featureSetHas(target.cpu.features, letter.feat)) {
+                            march_buf[march_index] = letter.char;
+                            march_index += 1;
+                        }
+                    }
+
+                    const march_arg = try std.fmt.allocPrint(arena, "-march={s}", .{
+                        march_buf[0..march_index],
+                    });
+                    try argv.append(march_arg);
+
                     if (std.Target.riscv.featureSetHas(target.cpu.features, .relax)) {
                         try argv.append("-mrelax");
                     } else {
                         try argv.append("-mno-relax");
                     }
+                    if (std.Target.riscv.featureSetHas(target.cpu.features, .save_restore)) {
+                        try argv.append("-msave-restore");
+                    } else {
+                        try argv.append("-mno-save-restore");
+                    }
                 },
                 else => {
                     // TODO