User mode support for Linux ELF files with no section header

In user mode Linux, Qemu currently refuses to load ELF files that do not
contain section headers (ehdr->e_shentsize == 0). Since section headers are not
required in order to load an ELF file, simply removing the e_shentsize check in
elf_check_ehdr() allows ELF binaries with no section headers to be run properly
in user mode:

Signed-off-by: Craig Heffner <cheffner@tacnetsol.com>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
