[GOLD] Increase --split-stack-adjust-size

For functions with small (< 256 bytes) stack frames, the current x86
do_calls_non_split ignores --split-stack-adjust-size and, in
combination with __morestack_non_split, supplies a non-split-stack
function with at least 0x100000 (1M) available stack.  On powerpc64, a
default of 0x4000 is not large enough to reliably work with the golang
testsuite.  This increase the default size to the defacto x86 value.

	* options.h (split_stack_adjust_size): Default to 0x100000.