Make source compile under Linux X32 ABI

The X32 ABI uses 32-bit pointers in combination with all
other features of x86-64 to save memory. This requires two
minor changes to x86 inline assembly macro checks, where we
need check for __LP64__ (64-bit pointers) rather than
__x86_64__ (which is set for X32). The resulting binaries
run approximately 2% faster, but since it requires all
dependent libraries to be available in X32 format it is
not something we will enable with a simple setting for now.
To use it manually, set CFLAGS and CXXFLAGS to "-O3 -mx32".

Fixes #1263.

Change-Id: I2555b2c68d4bba24f4e581268c472a47fe7d1531