Fix sysroot for standalone GYP build on ARM.

Fix sysroot for standalone GYP build on ARM. 

When compiling C files with deps on external libraries, seeing error like this:

CC(target) out/Debug/obj.target/platform/native_client/src/shared/platform/nacl_global_secure_random.o
In file included from ./native_client/src/shared/platform/nacl_secure_random.h:22,
                 from ./native_client/src/shared/platform/nacl_global_secure_random.h:14,
                 from native_client/src/shared/platform/nacl_global_secure_random.c:11:
./native_client/src/shared/platform/linux/nacl_secure_random_types.h:32:26: error: openssl/aes.h: No such file or directory
Committed: https://src.chromium.org/viewvc/native_client?view=rev&revision=8131

[robertm, 2012-03-29 14:03:53]

LGTM

Please augment the description with a short description of the
problems you see without this change, so we can search for it if we get hit by
it again.