jimtcl: restrict memory leak workaround on Linux only

jimtcl: restrict memory leak workaround on Linux only

The workaround for jimtcl 0.80 in commit 36ae487ed04b ("jimtcl:
add temporary workaround for memory leak in jimtcl 0.80") issues a
compile time error on macOS:
	../src/helper/command.c:157:22: error: aliases are not
	supported on darwin
	__attribute__((weak, alias("workaround_createcommand")));
The OS is x86_64-apple-darwin19.6.0 and the compiler used is
x86_64-apple-darwin13.4.0-clang.

Restrict the workaround on Linux host only. The fix for 'expr'
syntax change is already merged and the workaround will be dropped
soon.

Change-Id: I925109a9c57c05f8c95b70bc7d6604eb1172cd79
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reported-by: Adam Jeliński <ajelinski@users.sourceforge.net>
Fixes: 36ae487ed04b ("jimtcl: add temporary workaround for memory leak in jimtcl 0.80")
Fixes: https://sourceforge.net/p/openocd/tickets/304/
Reviewed-on: http://openocd.zylin.com/6241
Tested-by: jenkins