Merge branch 'alloca' into 'master'

Implement alloca.h

See merge request redox-os/relibc!180

include/alloca.h

+#ifndef _ALLOCA_H
+#define _ALLOCA_H
+
+#define alloca(size) __builtin_alloca (size)
+
+#endif /* _ALLOCA_H */

tests/Makefile

 # Binaries that should generate the same output every time
 EXPECT_BINS=\
+	alloca \
 	args \
 	arpainet \
 	assert \

tests/alloca.c

+#include <alloca.h>
+#include <stdio.h>
+#include <string.h>
+
+int main(int argc, char ** argv) {
+    char *str = (char *) alloca(17);
+
+    memset(str, 'A', 16);
+    str[16] = '\0';
+
+    printf("%s\n", str);
+}

tests/expected/alloca.stdout

+AAAAAAAAAAAAAAAA