Add baudrate constants

This adds the missing baudrate constants for linux where I've used them
directly.

lib/std/os/bits/linux.zig

@@ -1837,6 +1837,39 @@ pub const tcflag_t = u32;
 
 pub const NCCS = 32;
 
+pub const B0 = 0o0000000;
+pub const B50 = 0o0000001;
+pub const B75 = 0o0000002;
+pub const B110 = 0o0000003;
+pub const B134 = 0o0000004;
+pub const B150 = 0o0000005;
+pub const B200 = 0o0000006;
+pub const B300 = 0o0000007;
+pub const B600 = 0o0000010;
+pub const B1200 = 0o0000011;
+pub const B1800 = 0o0000012;
+pub const B2400 = 0o0000013;
+pub const B4800 = 0o0000014;
+pub const B9600 = 0o0000015;
+pub const B19200 = 0o0000016;
+pub const B38400 = 0o0000017;
+pub const BOTHER = 0o0010000;
+pub const B57600 = 0o0010001;
+pub const B115200 = 0o0010002;
+pub const B230400 = 0o0010003;
+pub const B460800 = 0o0010004;
+pub const B500000 = 0o0010005;
+pub const B576000 = 0o0010006;
+pub const B921600 = 0o0010007;
+pub const B1000000 = 0o0010010;
+pub const B1152000 = 0o0010011;
+pub const B1500000 = 0o0010012;
+pub const B2000000 = 0o0010013;
+pub const B2500000 = 0o0010014;
+pub const B3000000 = 0o0010015;
+pub const B3500000 = 0o0010016;
+pub const B4000000 = 0o0010017;
+
 pub const IGNBRK = 1;
 pub const BRKINT = 2;
 pub const IGNPAR = 4;