Change gcc_target_options to return std::string

Change gcc_target_options to return std::string

This patch was inspired by a recent review that recommended using
std::string in a new implementation of the gcc_target_options gdbarch
function.  It changes this function to return std::string rather than
an ordinary xmalloc'd string.

I believe this caught a latent memory leak in compile.c:get_args.

Tested on x86-64 Fedora 29.

gdb/ChangeLog
2019-10-15  Tom Tromey  <tromey@adacore.com>

	* gdbarch.h, gdbarch.c: Rebuild.
	* gdbarch.sh (gcc_target_options): Change return type to
	std::string.
	* compile/compile.c (get_args): Update.
	* nios2-tdep.c (nios2_gcc_target_options): Return std::string.
	* arm-linux-tdep.c (arm_linux_gcc_target_options): Return
	std::string.
	* aarch64-linux-tdep.c (aarch64_linux_gcc_target_options): Return
	std::string.
	* arch-utils.c (default_gcc_target_options): Return std::string.
	* arch-utils.h (default_gcc_target_options): Return std::string.
	* s390-tdep.c (s390_gcc_target_options): Return std::string.

Change-Id: I51f61703426a323089e646da8f22320a2cafbc1f