Fix comment parsing in interp_simple.c

[bcran]

Summary

loader.rc has comment lines without a trailing space, which get
interpreted as commands. Avoid this by only matching against the
backslash character.

Test Plan

Without the change, the following happens if bootx64.efi is loader_simp.efi

Trying: PciRoot(0x0)/Pci(0x1F,0x2)/Sata(0x0,0xFFFF,0x0)/HD(2,MBR,0x90909090,0x10419,0xB6E90)
Setting currdev to disk0p2:
> \
\: unknown command

/boot/kernel/kernel text=0x16a8a85 data=0x1c88f8+0x8191f8 syms=[0x8+0x17fe20+0x8+0x19ccce]

With the change, the loader gets further until failing, reporting an unknown
command "only" from the line "only forth definition" in e.g. stand/forth/loader.4th