Missing `std.os.linux` arch bits

[alexrp]

We're currently missing arch bits for some targets:

zig/lib/std/os/linux.zig

Lines 32 to 51 in e62aac3

	const arch_bits = switch (native_arch) {
	.x86 => @import("linux/x86.zig"),
	.x86_64 => @import("linux/x86_64.zig"),
	.aarch64, .aarch64_be => @import("linux/aarch64.zig"),
	.arm, .armeb, .thumb, .thumbeb => @import("linux/arm.zig"),
	.hexagon => @import("linux/hexagon.zig"),
	.riscv32 => @import("linux/riscv32.zig"),
	.riscv64 => @import("linux/riscv64.zig"),
	.sparc64 => @import("linux/sparc64.zig"),
	.loongarch64 => @import("linux/loongarch64.zig"),
	.mips, .mipsel => @import("linux/mips.zig"),
	.mips64, .mips64el => @import("linux/mips64.zig"),
	.powerpc, .powerpcle => @import("linux/powerpc.zig"),
	.powerpc64, .powerpc64le => @import("linux/powerpc64.zig"),
	.s390x => @import("linux/s390x.zig"),
	else => struct {
	pub const ucontext_t = void;
	pub const getcontext = {};
	},
	};

Compare to syscalls:

zig/lib/std/os/linux.zig

Lines 123 to 151 in e62aac3

	pub const syscalls = @import("linux/syscalls.zig");
	pub const SYS = switch (@import("builtin").cpu.arch) {
	.arc => syscalls.Arc,
	.arm, .armeb, .thumb, .thumbeb => syscalls.Arm,
	.aarch64, .aarch64_be => syscalls.Arm64,
	.csky => syscalls.CSky,
	.hexagon => syscalls.Hexagon,
	.loongarch64 => syscalls.LoongArch64,
	.m68k => syscalls.M68k,
	.mips, .mipsel => syscalls.MipsO32,
	.mips64, .mips64el => switch (builtin.abi) {
	.gnuabin32, .muslabin32 => syscalls.MipsN32,
	else => syscalls.MipsN64,
	},
	.riscv32 => syscalls.RiscV32,
	.riscv64 => syscalls.RiscV64,
	.s390x => syscalls.S390x,
	.sparc => syscalls.Sparc,
	.sparc64 => syscalls.Sparc64,
	.powerpc, .powerpcle => syscalls.PowerPC,
	.powerpc64, .powerpc64le => syscalls.PowerPC64,
	.x86 => syscalls.X86,
	.x86_64 => switch (builtin.abi) {
	.gnux32, .muslx32 => syscalls.X32,
	else => syscalls.X64,
	},
	.xtensa => syscalls.Xtensa,
	else => @compileError("The Zig Standard Library is missing syscall definitions for the target CPU architecture"),
	};

• alpha
• arc
• csky
• hppa
• m68k
• microblaze
• mipsn32
• or1k
• sparc
• sh
• x32
• xtensa