syscall(3)'s return type should be long.

This doesn't require us to change any of the syscall implementations
because (a) the LP32 ones have sizeof(int) == sizeof(long) anyway,
which is how we never noticed this bug before and (b) the LP64 ones
all use a 64-bit register for the result (and for the syscall number
too).

Bug: https://code.google.com/p/android/issues/detail?id=73952
Bug: 16568314

(cherry picked from commit 21972b61ec)

Change-Id: Ifbc424be29e5650ec72a24df25dd35f24fdd5b3c

libc/include/sys/syscall.h

@@ -37,7 +37,7 @@
 
 __BEGIN_DECLS
 
-int syscall(int number, ...);
+long syscall(long number, ...);
 
 __END_DECLS
 

tests/unistd_test.cpp

@@ -20,6 +20,7 @@
 
 #include <errno.h>
 #include <fcntl.h>
+#include <limits.h>
 #include <stdint.h>
 #include <unistd.h>
 #include <sys/syscall.h>
@@ -212,6 +213,14 @@ TEST(unistd, read_EBADF) {
   ASSERT_EQ(EBADF, errno);
 }
 
+TEST(unistd, syscall_long) {
+  // Check that syscall(3) correctly returns long results.
+  // https://code.google.com/p/android/issues/detail?id=73952
+  // We assume that the break is > 4GiB, but this is potentially flaky.
+  uintptr_t p = reinterpret_cast<uintptr_t>(sbrk(0));
+  ASSERT_EQ(p, static_cast<uintptr_t>(syscall(__NR_brk, 0)));
+}
+
 TEST(unistd, alarm) {
   ASSERT_EQ(0U, alarm(0));
 }