Support uClibc for the tests compilations

There are two issues that don't allow ZFS to be compiled using uClibc.
`backtrace()`, and `program_invocation_short_name` as a `const`.
This patch adds uClibc to the conditionals in the same way there are
already for Glibc for `backtrace()`; and removes the external param
`program_invocation_short_name` because its only used here for the
whole project.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: José Luis Salvador Rufo <salvador.joseluis@gmail.com>
Closes #11600

cmd/ztest/ztest.c

@@ -132,7 +132,7 @@
 #include <libnvpair.h>
 #include <libzutil.h>
 #include <sys/crypto/icp.h>
-#ifdef __GLIBC__
+#if (__GLIBC__ && !__UCLIBC__)
 #include <execinfo.h> /* for backtrace() */
 #endif
 
@@ -556,7 +556,7 @@ dump_debug_buffer(void)
 static void sig_handler(int signo)
 {
 	struct sigaction action;
-#ifdef __GLIBC__ /* backtrace() is a GNU extension */
+#if (__GLIBC__ && !__UCLIBC__) /* backtrace() is a GNU extension */
 	int nptrs;
 	void *buffer[BACKTRACE_SZ];
 

tests/zfs-tests/cmd/xattrtest/xattrtest.c

@@ -44,11 +44,9 @@
 #include <sys/time.h>
 #include <linux/limits.h>
 
-extern char *program_invocation_short_name;
-
 #define	ERROR(fmt, ...)                                                 \
-	fprintf(stderr, "%s: %s:%d: %s: " fmt "\n",                     \
-		program_invocation_short_name, __FILE__, __LINE__,      \
+	fprintf(stderr, "xattrtest: %s:%d: %s: " fmt "\n",              \
+		__FILE__, __LINE__,      				\
 		__func__, ## __VA_ARGS__);
 
 static const char shortopts[] = "hvycdn:f:x:s:p:t:e:rRko:";