mbedtls_ssl_config, mbedtls_ssl_session: reorder fields
mbedtls_ssl_config, mbedtls_ssl_session: reorder fields

Move small fields first so that more fields can be within the Arm Thumb
128-element direct access window.

Keep the int section after the pointer section: moving int fields first cost
a few bytes on the reference baremetal-m0plus build.

The ordering in this commit is not based on field access frequency.

Results (arm-none-eabi-gcc 7.3.1, build_arm_none_eabi_gcc_m0plus build):
library/ssl_cli.o: 19687 -> 19543 (diff: 144)
library/ssl_msg.o: 24834 -> 24726 (diff: 108)
library/ssl_srv.o: 20562 -> 20462 (diff: 100)
library/ssl_tls.o: 20907 -> 20707 (diff: 200)
library/ssl_tls13_client.o: 7272 -> 7252 (diff: 20)
library/ssl_tls13_generic.o: 4721 -> 4705 (diff: 16)

Results (same architecture, config-suite-b.h + MBEDTLS_ECDH_LEGACY_CONTEXT +
MBEDTLS_ECP_RESTARTABLE):
library/ssl_cli.o: 2936 -> 2876 (diff: 60)
library/ssl_msg.o: 3080 -> 3068 (diff: 12)
library/ssl_srv.o: 3400 -> 3372 (diff: 28)
library/ssl_tls.o: 6730 -> 6658 (diff: 72)

Signed-off-by: Gilles Peskine <f805f64266d288fc5467baa7be6cd0ff366f477b@arm.com>

ARMmbed/mbedtls,NXPmicro/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls