ANDROID: pkvm: x86: Add ENDBR to exception handlers

In line with commit 8f93402b ("x86/ibt,entry: Sprinkle ENDBR dust")
add ENDBR to entry points of pKVM's exception handlers.

Without this, kernel panics with "Missing ENDBR: nmi_handler+0x0/0x40"
whenever a NMI occurs (unless CONFIG_X86_KERNEL_IBT is disabled).

Signed-off-by: Dmytro Maluka <dmaluka@google.com>

Bug: 349990461
Bug: 353937603
Test: kernel doesn't panic when NMI occurs, e.g. in cases when pKVM
prints lots of debugs to dmesg (e.g. due to host repeated attempting
to access a pVM memory) and dmesg is redirected to a serial console,
causing system stalls.

Change-Id: I8fad1782eb98bc8ad7b2c5c105f1351b11631207
Signed-off-by: Dmytro Maluka <dmaluka@google.com>