Generate assembler system call stubs via genrule.

There's no need to check in generated code.

Test: builds & boots
Change-Id: Ife368bca4349d4adeb0666db590356196b4fbd63
diff --git a/libc/SYSCALLS.TXT b/libc/SYSCALLS.TXT
index 772f0e7..e2ea065 100644
--- a/libc/SYSCALLS.TXT
+++ b/libc/SYSCALLS.TXT
@@ -22,7 +22,8 @@
 #
 #      - Each parameter type is assumed to be stored in 32 bits.
 #
-# This file is processed by a python script named gensyscalls.py.
+# This file is processed by a python script named gensyscalls.py, run via
+# genrules in Android.bp.
 
 int     execve(const char*, char* const*, char* const*)  all