Don't pass empty options to GCC

Don't pass empty options to GCC

On aarch64-linux, I noticed the compile command didn't work at all.  It
always gave the following error:

aarch64-linux-gnu-g++: error: : No such file or directory

Turns out we're passing an empty argv entry to GCC (because aarch64 doesn't
have a -m64 option), and GCC's behavior is to think that is a file it needs
to open.  One can reproduce it like so:

gcc "" "" "" ""
gcc: error: : No such file or directory
gcc: error: : No such file or directory
gcc: error: : No such file or directory
gcc: error: : No such file or directory
gcc: fatal error: no input files
compilation terminated.

The solution is to check for an empty string and skip adding that to argv.

Regression tested on aarch64-linux/Ubuntu 18.04/20.04.

gdb/ChangeLog:

2021-03-29  Luis Machado  <luis.machado@linaro.org>

	* compile/compile.c (get_args): Don't add empty argv entries.