Modify the .code16 macro so that the 16-bit code segment can get a
chance to be correctly merged with other (possibly 32-bit) code
segments, without keeping generating multiple .text segments with
different attributes and generating the corresponding LNK4078 warning.
This fixes the warning when compiling NTOSKRNL on MSVC:
"v86.S.obj : warning LNK4078: multiple '.text' sections found with different attributes (C0520040)".
GAS uses ".double" symbol for declaring floating-point constants and
".quad" symbol for declaring 64-bit numbers.
This is not compatible with our macro for MASM and introduces bugs.
Now 64-bit constants are supposed to be declared using ".quad" macro.
