asan: dlltool buffer overflow: embedded NUL in string

asan: dlltool buffer overflow: embedded NUL in string

yyleng gives the pattern length, xstrdup just copies up to the NUL.
So it is quite possible writing at an index of yyleng-2 overflows
the xstrdup allocated string buffer.  xmemdup quite handily avoids
this problem, even writing the terminating NUL over the trailing
quote.  Use it in ldlex.l too where we'd already had a report of this
problem and fixed it by hand, and to implement xmemdup0 in gas.

binutils/
	* deflex.l (single and double quote strings): Use xmemdup.
gas/
	* as.h (xmemdup0): Use xmemdup.
ld/
	PR 20906
	* ldlex.l (double quote string): Use xmemdup.