modules: mbedtls: use proper memory alignment on 64-bit platforms

This commit instructs mbedtls to use 64-bit alignment in its internal
memory allocation routines when targeting 64-bit platforms. By default
mbedtls uses 32-bit alignment regardless the platform, what may result
in misaligned memory accesses, possibly inducing access time overhead or
exceptions

Signed-off-by: Vladimir Graudt <vladimir.graudt@syntacore.com>