For old mmap syscall, when executing on amd64 or ia64, enforce the PROT_EXEC if prot is non-zero, process is 32bit and kern.elf32.i386_read_exec syscal is enabled. This workaround is needed for old i386 a.out binaries, where dynamic linker did not specified PROT_EXEC for mapping of the text.
For old mmap syscall, when executing on amd64 or ia64, enforce the
PROT_EXEC if prot is non-zero, process is 32bit and
kern.elf32.i386_read_exec syscal is enabled. This workaround is needed
for old i386 a.out binaries, where dynamic linker did not specified
PROT_EXEC for mapping of the text.

The kern.elf32.i386_read_exec MIB name looks weird for a.out binaries,
but I reused the existing knob which already has the needed semantic.

MFC after:	1 week

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