ARM: Use 8-byte alignment for DOUBLE/LONGINT

According to the ARM ABI and Ubuntu's arm-linux-gnueabihf-gcc, double and
long long (LONGINT) are 8-byte aligned, as on Win32, unlike 32bit x86
Linux.

Relevant ARM ABI document:
Procedure Call Standard for the ARM Architecture (AAPCS)
http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ihi0042e/index.html
makefile: Normalize TARGET_ARCH to "x86_64" in case it was set to "amd64"