i386: Generate 3-byte NOP for -mnop-mcount -m16

i386: Generate 3-byte NOP for -mnop-mcount -m16

When generating 16-bit code via -m16, the NOP mcount code generation
emits a 5-byte NOP. However, that is neither a valid i8086 instruction
(long NOPs are PentiumPro+), nor would it get decoded as a 5-byte
instruction. It's a 4-byte 'nopw 0(%si)' followed by a zero byte. The
latter causes the following instruction to get misinterpreted as some
form of ADD.

Fix this by emiting a 3-byte no-op 'lea 0(%si)' instead which makes it
compatible with systems lacking long NOP support.

Add a test for this and change the existing one accordingly.

gcc/ChangeLog:

	* config/i386/i386.cc (x86_print_call_or_nop): Fix 16-bit NOP
	generation.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/nop-mcount-m16.c: New test.
	* gcc.target/i386/nop-mcount.c: Adapt to test for 5-byte NOP.

Signed-off-by: Mathias Krause <minipli@grsecurity.net>