modpost: handle huge numbers of modules.

strace shows:
72102 execve("/bin/sh", ["/bin/sh", "-c", "echo '  scripts/mod/modpost -m -a
-o /cc/wfg/sound-compiletest/Module.symvers      -s'; scripts/
mod/modpost -m -a -o /cc/wfg/sound-compiletest/Module.symvers      -s vmlinux
arch/x86/crypto/ablk_helper.o arch/x86/crypto/aes-i586.o arch
/x86/crypto/aesni-intel.o arch/x86/crypto/crc32-pclmul.o
...
drivers/ata/sata_promise.o "...], [/* 119 vars */] <unfinished ...>
71827 wait4(-1,  <unfinished ...>
72102 <... execve resumed> )            = -1 E2BIG (Argument list too long)

So we re-run the shell command which produces the list and feed it into modpost -T -.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>