Consistently use -f elf instead of -f elf32

The freebsd platform file already did things like this.
Mark the x86 assembly object files as not requiring an executable stack

This avoids having to add extra linker flags in order to specify this.

This is similar to how this already is handled for the arm assembly.
Merge pull request #1215 from mstorsjo/x86-asm-noexec-stack

Mark the x86 assembly object files as not requiring an executable stack