load warning: cannot find entry symbol _start
Use the label _start instead of main for the ELF entry point. main implies it's like the C main function, but this isn't even a function (e.g. you can't ret).
You don't say, but from the error messages and code I assume you're building your 32bit code with nasm -felf32 hello32.asm && ld -melf_i386 -o hello32 hello32.o
(If you're actually building 64bit code, you're lucky that it happens to work, but it'll break as soon as you do anything with esp instead of rsp.)
The error message is from ld, not from nasm. It says so right in the message. Tim's comment is correct: ld looks for a _start symbol in the files it links, but sets the entry point to the beginning of the text segment if it doesn't find one.
It doesn't matter what other global/external symbols you define. main has no relevance at all here, and could point anywhere you want. It's only useful for a disassembly output and stuff like that. Your code would work exactly the same if you took out the global main / main: lines, or changed them to any other name.
Labelling that as main is unwise because the ELF entry point is not a function. It's not main(), and doesn't receive argc and argv arguments, and can't ret because ESP is pointing at argc instead of a return address.
Only use main if you link with gcc / glibc's CRT startup code that looks for a main symbol and calls it after initializing libc. (So functions like printf work. Technically dynamic linker hooks let libc initialize itself before your _start if you linked it, but generally don't do that unless you understand exactly what you're doing). Related: Assembling 32-bit binaries on a 64-bit system (GNU toolchain)
e.g. gcc -m32 -no-pie -o hello main.o if you do define a main:
instead of gcc -m32 -static -nostdlib -o hello start.o
(which is equivalent to your bare ld).
(For the past few years, Linux distros have configured GCC with -pie as the default, which wants position-independent code. But that's really inconvenient in 32-bit mode without RIP-relative addressing (look at GCC asm output for example), and means ld won't convert call printf into call printf@plt for you. So for most hand-written asm following most tutorials, you want traditional non-PIE executables so no text relocations are needed.)
I would suggest that you link your object files (however they are produced) with gcc, not ld.
gcc will call ld with the appropriate options, since it knows more about the source code and will create whatever is necessary for the assumptions that ld makes.