slightly optimize __brk for size

there is no reason to check the return value for setting errno, since
brk never returns errors, only the new value of the brk (which may be
the same as the old, or otherwise differ from the requested brk, on
failure).

it may be beneficial to eventually just eliminate this file and make
the syscalls inline in malloc.c.

src/malloc/__brk.c

@@ -3,5 +3,5 @@
 
 uintptr_t __brk(uintptr_t newbrk)
 {
-	return syscall(SYS_brk, newbrk);
+	return __syscall(SYS_brk, newbrk);
 }