Look for the character to locate before checking for the NUL character

As documented in PR port-i386/49208, this fixes strchr(s, '\0'), as used by
the FAT first-stage bootloader on x86 (bootxx_msdos

Look for the character to locate before checking for the NUL character

As documented in PR port-i386/49208, this fixes strchr(s, '\0'), as used by
the FAT first-stage bootloader on x86 (bootxx_msdos).
strchr(s, '\0') is otherwise equivalent to strlen(string), which would
probably look nicer in the original file, dosfs.c from libsa.

Confirmed working in qemu and on real hardware.
ok joerg@

XXX pull-up to netbsd-6 and netbsd-7

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For all i386 string assembly functions that don't overlap use END() so
that symbol size information is available.

Add support for size optimised versions of the assembler functions in
the small subdirectory. Provide more compact versions of the functions
used by bootxx in x86.