x86asm: Generate endbr32 based on __CET__.

x86asm: Generate endbr32 based on __CET__.

The VIA Nehemiah CPU is a x86-32 CPU without SSE2 support. It does not
support multi byte nops and considers the endb32 opcode as an invalid
instruction.

Add an ifdef around the endbr32 opcode on x86-32.

Fixes: #18334
Signed-off-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/18353)