gdb/x86: Fix `-Wstrict-overflow' build error in `i387_collect_xsave'

gdb/x86: Fix `-Wstrict-overflow' build error in `i387_collect_xsave'

Make `i' defined within `i387_collect_xsave' unsigned, removing a
`-Werror=strict-overflow' compilation error:

.../gdb/i387-tdep.c: In function 'void i387_collect_xsave(const regcache*, int, void*, int)':
.../gdb/i387-tdep.c:1348:1: error: assuming signed overflow does not occur when assuming that (X + c) < X is always false [-Werror=strict-overflow]
 i387_collect_xsave (const struct regcache *regcache, int regnum,
 ^
cc1plus: all warnings being treated as errors
Makefile:1610: recipe for target 'i387-tdep.o' failed
make: *** [i387-tdep.o] Error 1

seen with GCC 5.4.0, a commit 8ee22052f690 ("gdb/x86: Handle kernels
using compact xsave format") regression.  While `regnum' can be -1 on
entry to the function, to mean all registers, `i' is only used with
non-negative register numbers.

	gdb/
	* i387-tdep.c (i387_collect_xsave): Make `i' unsigned.