m68k: Uninline strchr()

Some versions of gcc replace calls to strstr() with single-character
"needle" string parameters by calls to strchr() behind our back.

If strchr() is defined as an inline function, this causes linking errors
like

	ERROR: "strchr" [drivers/target/target_core_mod.ko] undefined!

As m68k is the only architecture that has an inline strchr() and this
inline version is not an optimized asm version, uninline strchr() and use
the standard out-of-line C version in lib/string.c instead.

This also decreases the defconfig/allmodconfig kernel image sizes by a few
hundred bytes.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>