std.Target: make ultrasparc3 the baseline for sparc64-solaris

llvm/llvm-project#149990

Author: alexrp

lib/std/Target.zig

@@ -1852,6 +1852,10 @@ pub const Cpu = struct {
                 .riscv64 => &riscv.cpu.baseline_rv64,
                 .s390x => &s390x.cpu.arch8, // gcc/clang do not have a generic s390x model.
                 .sparc => &sparc.cpu.v9, // glibc does not work with 'plain' v8.
+                .sparc64 => switch (os.tag) {
+                    .solaris => &sparc.cpu.ultrasparc3,
+                    else => generic(arch),
+                },
                 .x86 => &x86.cpu.pentium4,
                 .x86_64 => switch (os.tag) {
                     .driverkit => &x86.cpu.nehalem,