elf32-nds32: Don't define fls if it is provided by the system

elf32-nds32: Don't define fls if it is provided by the system

The fls function already exists on macOS and FreeBSD (and probably others),
leading to this error:

/Users/simark/src/binutils-gdb/bfd/elf32-nds32.c:5074:1: error: static declaration of 'fls' follows non-static declaration
fls (register unsigned int x)
^
/usr/include/strings.h:87:6: note: previous declaration is here
int      fls(int) __OSX_AVAILABLE_STARTING(__MAC_10_5, __IPHONE_2_0);
         ^

Add a configure-time check for it, and only define it if the system doesn't
provide it.

bfd/ChangeLog:

	* configure.ac: Check for fls.
	* elf32-nds32.c (fls): Only define if !HAVE_FLS.
	* config.in: Re-generate.
	* configure: Re-generate.